Well, most seats are sprung in some manner or another. If the spring is bottomed out, though, it won't help!
A suspension seatpost has a spring, and a shock absorber built-in, so that the stresses are averaged out. If it's just a spring, it may help the backside, but, it may not help the frame as much as the seatpost would. (in a car with bad shocks, you can bounce up & down more...)