The sartorius muscle is a bit of a mystery to me. I am thinking that the best explanation for it is to compensate for side effects of using the iliotibial band to balance the body sideways while only one leg is on the ground. The iliotibial band is great for that, but, maybe it as a side effect tends to laterally rotate the knee during part of the gait cycle. To compensate for that, the sartorius muscle antagonistically medially rotates the knee. This lets iliotibial band be used to balance the body.

Without being an expert, sartorius seems to have evolved in humans more towards for some function that ancestral sartorius did not provide, it seems to originate more medially in chimpanzees and even more medially in horses. The iliotibial band is uniquely human, if I understand right, an an adaptation for bipedalism in humans (Eng, 2015). So, the idea is iliotibial band as adaptation to balance body in bipedal gait, via lateral side of leg, is compensated for with sartorius at medial side.

The gluteus muscle inserts on the ilitibial band, and will laterally rotate the lower leg. The extension at the hip joint in that same movement, will stretch the sartorius muscle, and medially rotate lower leg.

And obviously, it can be ascribed hip lateral rotation, flexion, abduction, and so on, but, it never felt like why it is there. And protecting the femoral artery and nerve in adductor canal, but, also does not feel like main reason. And, possibly tightening fascia as Tom Myers said, like leech on sail boat, but also does not feel like why it is there. Antagonist for gluteus maximus iliotibial tract seems like perfect explanation.

Synapses

Eng, C. M., Arnold, A. S., Biewener, A. A., & Lieberman, D. E. (2015). The human iliotibial band is specialized for elastic energy storage compared with the chimp fascia lata. Journal of Experimental Biology, 218(15), 2382–2393. https://doi.org/10.1242/jeb.117952