The question could be formulated in different ways. If, for instance, one does not believe in a creatio e nihilo, one will rather ask why does God allow the persistance of the world or just "why something and not nothing?". But I am sure that the question becomes particularly relevant if one adopts the creationist perspective, so that God must have actively decided to create the world, a decision which S/He could have avoided and which was completely in His/Her hands, since there was no reality such as matter independent of Her/Him.
In a comment to this post, Roger suggests that "bringing glory to God is the highest good--and God's purpose for doing anything at all".
Now, I can see that this seems in harmony with so much theological thought. However, I cannot be convinced by it.
To me, this seems too close to the idea of an egotic God, who wants to be celebrated. An alternative reading would be the pantheistic view that there is nothing but God and what brings glory to Her/Him brings glory to everyone else.
If I were to look for a solution to the initial question, though, I would rather imagine that God created us because S/He missed us. S/He could not avoid feeling a love which went beyond Him/Herself (yes, I know that this points to God's incompleteness, but I prefer a God who is charitas to a God who is an Aristotelic Perfect Being).