Painting by William Arkle - this blog post is also based on Arkle's insights
This question (why some-thing, why every-thing; rather than no-thing) is - or ought to be - uniquely easy for Christians to answer; compared with any other religion that posits a creator-god.
The question arises from trying to understand why a god that is powerful enough to create everything should have any reason for doing so; since presumably such a god would be utterly self-sufficient.
The Christian answer relates to such facts that all men are Sons of God-the-creator; that God-the-creator loves all Men; and that Jesus was divine, was the ('only begotten') Son of God, and he told his disciples that in knowing Jesus they also knew his Father.
Thus: we are like God; God is like me and you and anyone else. In vital ways, God's primary motives are therefore understandable by you and me and anyone else. From this, we can infer that God created everything, including men and women, for the same basic reason that you or I or anyone else would create such a universe.
So if we imagine ourselves as God before creation (whether you regard God as a single being of no sex, or - as I do - regard God as a dyad of man and woman, bound eternally by love) the purpose of creation is 'for' making other gods, beings qualitatively like oneself - drawn and held together by eternal loving relationships, and working together - mutually aligned by this love - on the endless business of creation.
(God's creation just-is this state of loving creativity. Thus, creation is Not about 'making stuff' - it is instead establishing this 'process' of beings creat-ing.)
God wants a family, wants friends, wants others of his kind: the same kind as the Father and the Son, and more such people would be better: Sons of God bound by love, and doing the work of creation.
Why this - rather than eternal solitude and no creation? Do you really need to ask? To answer, all you need to do is imagine, to empathically-identify-with, to understand how-God-felt in that primordial divine situation.