Why did God rest after the six days of creation?

I don’t know, ultimately, the answer to that. But what I do see in that account is this: after the six days of creation, God rested on the seventh day and hallowed it. This says to me that the ultimate purpose of creation is holiness.

We tend to focus our attention on the sixth day, on the creation of human beings. That’s the penultimate purpose. The ultimate purpose is that everything He creates in the first six days will redound to His honor, His glory, and the communion of His holiness.