What vital control capability do liquid propellant systems primarily offer over solid rocket motors once ignited?

Throttling and engine restart capability

Liquid propellant systems provide crucial operational flexibility that solid motors inherently lack. Because liquid propellants (fuel and oxidizer) are stored separately and pumped into the combustion chamber, their flow rate can be precisely managed. This ability to adjust the flow is called throttling, allowing engineers to control the rate of thrust produced by the engine. More critically, liquid systems allow for the engine to be completely shut down and subsequently restarted, which is essential for performing complex trajectory corrections, orbital maneuvers, or emergency abort scenarios during flight. Solid motors, once ignited, burn until all propellant is consumed, offering absolutely no mid-burn control.