"In the beginning was the Word, and the Word was with God, and the Word was God." (John 1:1)
We understand that "the Word" is Jesus. And you are asking why Jesus is called "the Word".
This term, Word, could be an image to represent something about Jesus. A human word starts in our mind and heart, then it proceeds from us in order for it to produce a certain result.
Like this, Jesus before the earth was, was in God's heart, and He was God, too. And He proceeded to this earth in order to produce God's result, which is that there will be many people who are like Jesus > Romans 8:29.
God did not send only a word composed of letters in order to do this, but He sent His own Son as the model, the image of how our Father wants us all to be like His own Son whom He so loves and enjoys . . . so we can be pleasing to Him the way Jesus is. And we can share in our Father's love the way Jesus does, and Jesus has claimed this in prayer for us > John 17:22-26.
Also, I consider how Isaiah 55:11 says >
"'So shall My word be that goes forth from My mouth;
. It shall not return to Me void,
. But it shall accomplish what I please,
. And it shall prosper in the thing for which I sent it.'"
. . . . . . . . . . . . . . . . . . . . . (Isaiah 55:11)
Possibly, here, God means both His words and Jesus shall do all which God means by giving us His scriptures and sending His own Son Jesus.
And Jesus is the living meaning of all which is written by God or spoken by God in words. And how Jesus has lived and loved is the meaning of all which God has given us for words. So, in this way Jesus is the Word of God.
And, by the way, then, this can mean that our example can be used by God as His message to people. The example of Christ and His example through us can minister His deeper-than-words meaning of His words.