A different kind of damage
Traditional low kicks to the thigh cause bruising that is painful but slow to meaningfully limit movement. The calf kick, aimed lower at the peroneal nerve running along the outside of the shin, produces a different effect: repeated strikes there can numb the leg and compromise a fighter's ability to plant weight on it, which affects both their striking power and their takedown defence far more directly than a thigh kick does.
A favourable risk trade
Thrown from range with a low kicking motion, the calf kick exposes the kicker to less counter-striking risk than a head kick, which requires more commitment and balance to throw. Because it can be thrown repeatedly without much cumulative risk to the kicker, fighters can afford to use it as a jab-like tool throughout a fight rather than saving it for a single opportunity.
The cost of defending it
Checking a calf kick, by lifting the shin to block it, requires a stance adjustment that can itself slow a fighter's footwork and readiness to counter. Fighters who check every kick correctly avoid the damage but often trade away some of their own offensive rhythm to do so, which is why opponents facing a good calf kicker often look tactically cautious rather than merely hurt.