Box Spring - necessary?

Are box springs really necessary when buying a spring mattress? The
manufacturer and sales people say they're important because it acts as
a shock absorber and the mattress will last longer. But I'm not sure I
believe this. Just because the manufacturer and sales people say this,
it doesn't mean it's true. It seems to me they'll say anything to sell
the product and make money.

Is there any proof that a box spring works? Futon mattresses don't
require a box spring so why do spring mattresses? What's wrong with
just using plywood? My main concern is that if I buy a mattress, I want
it to last as long as possible. Will using plywood instead of a box
spring decrease the life of a mattress?

I have to wonder if box springs are like those 3 year service plans
that stores like Best Buy try to sell you when you buy a new computer.
They act like it's extremely crucial to buy the service plan, like you
might die if you don't. But it's very unlikely that your computer will
crap out within three years of buying it. By purchasing the plan,
you're most likely wasting your money. Consumer reports have even
called them a scam. So it certainly makes me question whether box
springs are important or not.