Any reason a composer does this?

Yes, that's the case here. The notation is trying to suggest the Bb's in that first measure all be taken by the LH (ditto for the notes repeated in various octaves in the other measures). So without the clef changes, there would have been crazy ledger lines required. No one would want to read that. The only other alternative would be to redistribute the notes between the staves so all the high notes were in the top staff, low notes in the bottom, and then add explicit "LH" and "RH" markings all over the place to tell you which notes to play with which hand., That would be harder to read. So the notation used here is definitely the best way to go by far.

FWIW, while the choice here may have been a no-brainer, these sorts of decisions may be made one way or another by the composer in his personal manuscript, but it's the editor of the particular published version you are looking at that makes the final call on this kind of things. Composers are trained/skilled in music, editors in notation, and these don't always overlap as much as you might think. So an editor will often choose to notate something differently from how the composer's handwritten manuscript may have appeared, to make it notationally correct / clearer.