Why does the Shuttle have Solid Rocket Boosters?

[Ian Curtis]

Alrighty, I've been searching all over the internet over the past few days, and can't find a satisfactory answer to this question.

Why does the Shuttle have Solid Rocket Boosters? And why do they provide more thrust than the External Tank? I was always under the impression that there was more thrust to be had in liquid rocket propellants as compared to solid ones.

I looked it up, and one potential reason I found was that the spent SRBs are simple (relative to liquid boosters, that is) and so can survive the fall into the ocean better. A downside to solid propellants is you can't turn them off once you start them, so if something starts going wrong, you've got to get out quick.

If the Solid Rocket Boosters can provide so much thrust, how come there aren't simply three of them to send the Shuttle skyward? And why do most (all?) launch vehicles use liquid stages when clearly so much thrust can come from solid propellants?