The shuttle used them as early-flight boosters. Same trick used by Ariane V, Titan III / IV, Delta II / III / IV, endless other big vehicles... They are much

The shuttle used them as early-flight boosters. Same trick used by Ariane V, Titan III / IV, Delta II / III / IV, endless other big vehicles... They are much cheaper to develop per thrust than liquids (They are expensive per size, but scale up to higher thrusts well), and you often want most of your thrust early on rather than later. Solid boosters you ditch after 30s or a minute raise your T/W ratio so you don't loose as much fuel to gravity losses, and get your more efficient but less powerful liquids into the upper atmosphere. Both Ariane V and Shuttle typify this, with almost all liftoff thrust from the solids, and a small, expensive hydrogen-burning engine continuing to most of orbital velocity. For the shuttle, this was aimed at reusablility, they could do most of their work on expensive engines, and not need to pay that cost every flight. Ariane did it so it could make near-orbit with only engines it lit on the pad, for reliability sake. (Planned for the Hercules spaceplane, which wouldn't have used the upper stages that satellites would eventually use.)