I've been doing a lot of knitting lately. It's occurred to me that when one wants to just relax and watch some TV or a movie, knitting while you do it transforms it from pure entertainment to a pleasant but useful activity, without making it less enjoyable. There is of course the cost of the materials, but they're not so great (especially if you knit mostly socks, like I've been doing lately).
It's a bit like hybrid gasoline/electric cars (except for the toyota prius and the honda insight). By adding some extra components, the hybrids get better fuel efficiency without sacrificing anything.
Knitting while you relax lets you get more out of your relaxing time without really sacrificing anything.
There is, of course, no such thing as a free lunch. But sometimes there does seem to be some awfully cheap lunches available.