Why did Michael...? (Spoiler!)

There are some important clues provided during this scene. So, Kaufman inserted it as a twisted joke to wrap up the ending. I thought it was very funny and clever and unexpected, because all normal behavior would hide it.

On the surface, yep, pretty idiotic.
I wonder if the gift was maybe a symbol of the cyclical nature of mental illness - that whatever troubles Michael will inevitably be passed along to his son the way these things tend to do.