For variety (and in my case so I can dedicate one bike set up for commuting and one to recreational without having to constant switch lights, bags, etc. between them.)

My flat commute is equally enjoyable and the same speed and effort with or without a freewheel.

The difference to me was more to do with having one gear (which a SS does as well) than the freewheel and that difference helped build up leg strength from starts and leg speed for cruising. But the OP of course already has this advantage.

I don't however get the idea of developing smoother spin on fixed, it seems a bit harder to spin smooth with a freewheel as the fixed forces one to, so when I go back to freewheel it takes a few days to even things out again.

Al