Here is the best explanation I have heard:
McCracken says most homes are consolidating around a two-hub model. A
PC (or Mac) with some multimedia features anchors the home office,
while a TV with some computerized gear–think TiVo, not desktop
computer–owns the living room. Tech marketers talk about the "2-foot interface"
of the PC versus the "10-foot interface" of the TV. When you use a
computer, you want to lean forward and engage with the thing, typing
and clicking and multitasking. When you watch Lost, you want
to sit back and put your feet up on the couch. My tech-savvy friends
who can afford anything they want set up a huge HDTV with TiVo, cable,
and DVD players–then sit in front of it with a laptop on their knees.
They use Google and AIM while watching TV, but they keep their 2-foot
and 10-foot gadgets separate.
Lean forward, lean back. They are two pretty different angles. Imagine if that distinction were to drive the development of entertainment media over the next century. Here is the full argument. By the way, when I lie in bed I find I have completely different thoughts depending on whether I am on my side or on my back. And I hate to read when I am leaning on my elbows.