The roundhouse kick is generally used in varying levels (low, middle, high) to inflict damage to different parts of the opponent's body. Low kicks generally target the thigh just above the knee joint for the purpose of weakening the leg, which serves many purposes such as limiting the opponent's mobility, potentially his ability to stand on the leg altogether and his ability to use it for forceful kicks. Fighters are taught to repeatedly strike the same point on the thigh to increase the cumulative effect of low kicks as a match proceeds.
Muscles of the Thigh
The first target is actually four targets in one, the quadriceps femoris muscle group. The Quads. Individually, the quads consist of the rectus femoris, vastus intermedius, vastus medialis and the vastus lateralis.
An effective leg kick to these muscles, trapping the muscle between the shin and your oppnents femur bone, will slowly degrade their base, deadening the legs and opening them up for body and high kicks in later rounds.