Personally I believe often it is the only time you can truly learn. Often you are being paid to do a job, and in my experience (agency work) that job has been the same thing day in, day out, rarely properly learning new cool stuff to play with.

I wrote an article on this recently.

What’s the most important thing to know as a developer? http://sydl.me/1thingtoknow

I want to keep learning new stuff... the only way to learn is by playing with it. So I create stupid little sites quite often http://instac.at http://isitherapture.com http://obamasmessagetoamerica.com

The list goes on...