I might tend to agree with Ajax ...
The devil's most basic "grounds" would be that Job is a "human being" and capable of sin. There need not even be more grounds than that. However the Accuser pointed out that Job had a lot of favor, and if that favor were taken away, then Job would show his "true nature" and curse God, etc.
But even after everything of Job's was struck --- things of his world, his flesh, the flesh of his loved ones --- Job's *spirit* showed it's nature. And this is important to see.
Where as I don't know if God deliberately "goaded" the Accuser, Job is a fine example of why men will judge angels, and the Accuser is ultimately condemned and we can partake with the Lord in His ruling of His Kingdom.
Human beings, are lower than the angels. Spirits trapped in flesh, given over to the flesh. So we have factors working against us. Angels ARE spirit, and not trapped in the flesh. And the Accuser who brings accusation against us, continually uses God's own words, our own natures, and "truths" against us, which if you're being "fair", can be considered as valid arguments, given the circumstances. Furthermore, the Accuser is capable of drawing spirits into perdition ... so that they become slaves to the flesh. So it's not as though Satan doesn't have a "leg to stand on". He's got evidence, experience, and spirits that have fallen over to fleshly lusts and evil to show that not only are humans weak willed, but so are angels, and he can "prove it".
But where he falls short, is with people like Job, who prove that their spirit can still belong to God's, and that they are, in fact, CHILDREN OF GOD. Strike them, in their weakest moments, where it hurts the most, and their spirits will still stay safe with their Lord. And this is important, because it shows how beautiful, and amazing, human beings really can be, and provides EVIDENCE AGAINST THE ANGELS AND SATAN.
So when given the opportunity, the spirit of angels who fell, choose flesh, and don't come back. The spirit of Satan, chooses condemnation and judgement and evil, death, destruction, deception, and bloodshed, and doesn't relent. But the spirit of man ... the elect, Children of Light, Children of God, brethren of Christ ... are *subject* to the flesh, but ultimately choose Life, peace, faith, joy, love, and the Spirit of the Creator, *even when they are tested* in both areas. Thus, we will judge the very angels, and "spirits", that condemned us, sought our destruction, and tried to tear us down ... but were wrong in their assessment that we could be overcome. Not all of us turned out like them, which they thought we would.
So I don't think it's as though God needed Job to build a case against Satan and the Sons of God ... I think Job had an enemy, and God not only ultimately gave Job empowerment over his enemy, but God gave Job the right to stand in judgement over his enemy ... an enemy who was more powerful, and had a "higher place" than Job. So we can trust Job's perception, and rulings, on Justice ... because he was given a gun to commit murder, so to speak, but did not commit murder. Instead, the one who gave him the gun to "set him up" will go down for attempted murder, with Job as a "witness". And the more Job's we have, the more empowered our Godly Family is, as we fill the Kingdom and evil has no place there, because even it's arguments against us can be proven wrong.
Peter is another example. Satan wanted to sift him as wheat, and did ... but Peter passed. Did he deny the Lord ? Of course. But his spirit stayed the same ... and when given the chance, he endured, and stayed with God, revealing the difference between the Children of God and others. Same with David after he committed his atrocity with Bathsheeba. Same with John the Baptist when he was considering denouncing Jesus as Lord. Their spirits showed why they are the elect, and will judge angels, whom they were made "lower than". Even when our advantages are taken away, and our spirits are compared side by side to the spirits that fall ... we prove their accusations about us incorrect, even when we still have the flesh working against us, which they did not have.
* For the record, I'm not really trying to define the "difference in spirits" here, just using general terms to get a point across I think ...