one reason is because it attracted more souls to him. another reason is that humans die. another reason is because people who are evil do evil things to others.
A third reason is to teach the lesson - directly and in person - that death is nothing. WE, who fear and cringe at death, will compromise our morals and what we know God wants, in order to preserve ourselves. And we do that more than just life and death - truth is, few of us are exposed to life and death. We compromise over money, status and pleasure, fear of loss.
The ULTIMATE loss, to human eyes, is death. People will do ANYTHING to avoid that, especially people who don't trust (or believe in) God.
Jesus had to die SO THAT he could rise from the dead and thereby DEMONSTRATE that, yeah, God is the King over death as well, that at God's command, the dead rise. Suddenly, death ceases to be the end - IF (IFF) one is with God and follows Jesus. Death has lost its sting, if one sees clearly.
Jesus had to die, as a man, in part so that men would see a man die - and die at the hands of the most powerful authorities on earth: Church AND State, together. And then that man just simply rise from the dead. The most powerful human entities DID THEIR WORST, to just a single man, a nobody, and HE walked out of the tomb - The Roman Empire and the Jewish Temple and King Herod, all combined, were unable to destroy a nobody carpenter. The stripped him of everything, and they killed him, and God LAUGHED at them and the man walked right out of the tomb, demonstrating that, actually, the superpower government AND the religious authorities in all of their might AND the local police AND all of the human laws and the Laws of Nature and Death themselves have NO POWER AT ALL to bind God. They can pronounce and inflict death, and God simply throw them aside, with a big heavy stone, and bring a man back to life.
And Jesus had to be publicly tortured to death, in the most fearful way, to give the example that a man who really UNDERSTANDS that the Law of Nature itself, the laws of Physics and Biology, AND the laws of the superpower state, and the laws of pain and fear, can all be overridden by the CHOICE of a simple flesh and blood man to defy them all.
Jesus was tortured to death, and did not recant, and he walked out the tomb and destroyed the church of the time (the Temple and the entire priesthood are GONE FOREVER - all that's left is the religious debating society of the Pharisaic rabbis which call themselves "Judaism", but which do not have the power to actually obey the Sinai Covenant, because God stripped of them both the necessary altar and the necessary priests; the Roman Empire is also utterly gone. In fact, the Roman Empire fought Christianity for three hundred years, and in the end, the Empire lost. The Christians never bore arms. They died, in blood and agony, but they all went to heaven. Their torturers will peer up from the flames: their LAW was destroyed, their STATE was destroyed, their ARMY was destroyed, and they individually, their bodies and spirits, were destroyed. They fought God by destroying men - torturing and killing me - but the men were all restored everything, while THEY, the mob, the army, the mass, the empire, the law, the superpower, were ground into dust, the idols wiped away, but the MEN who worshipped the idols died too, but then got to live with the consequences of their defeat for the same endless woe they sought to inflict on the Christians who tried to save them).
It was important that Jesus be a man - for man to know that God knows what it is like to be a man.
It was important the Jesus be humiliated and tortured by both "church" and state, all levels of the state, that he be formally judged under the laws of three sets of men, including - importantly - the holy laws of the Temple and its prophet priest - the TEMPLE had to fail in the final instance, and condemn God to death, so that God could justly destroy it for good and thereby put an end to THAT covenant (by making it impossible to follow anymore).
It was important that he be publicly, visibly, shockingly and clearly KILLED, that he be dead as dead can be.
SO THAT when he walked out of tomb alive and rejoined his friends, the utter impotence and toothless of the Roman Empire, the Roman Emperor, the Roman Army, the Roman governor, the Jewish King, the Jewish High Priest, the Jewish High Court, the Jewish Temple authorities, and the mob were all demonstrated. They all stood against God by destroying a man utterly. And they were all utterly unable to do it. All of those entities of that time are gone from the earth, but the people of Jesus remain, and we get to conquer death and live forever because he did.
The modern equivalent would be a man standing up to the United States and Russia, and being destroyed by a joint atomic bomb strike. And then his atoms reassembling and he walk out of the fire, rendering their nukes inert, and demonstrating that the combined might of the world is nothing, because the life of Christians is superior in durability to the laws of physics.
THAT'S what Jesus' death and resurrection demonstrates in the flesh and, if we trust him, what is promised to us: we are immortal and utterly invincible, even against the exploding of the sun and black holes, let alone mere men and rabies. Invincible, immortal and eternal - that is what we ARE, if we choose to be. The only way to that, though, is by obeying Christ. God made HIM that way, publicly, so that we know that if we do as he said to do, we can have the same reward (of life and imperishability, not of rulership over the universe, of course - we can be LIKE Christ, we can't BE Christ).
Christianity grants immortality superior to the power of the Laws of Nature. Christ's very public crucifixion and death, and his resurrection (immortalized for the public forever, by God, in the Shroud of Turin's evidence), proved it. God did that for Christ. If we TRUST Christ, he said that God will do that for us also. And we probably won't have to be scourged and crucified to obtain it. When we are bitten by the sting of sin's serpent, we must look to OUR "brazen serpent", which is Christ crucified and then risen, and we will live. Not in a mystical sense - a literal sense. Christ promises us immortality, just like his. Like him, we have to pass through death to get there.
Only death? So what?