When you train legs you are working your posterior chain and helping with everyday functional movements. Bending and squatting are part of our everyday life. It's very likely that even on leg day you are training more than only your legs. Through compound exercises like squats, deadlifts, and good mornings, you'll be working your arms, core, back, shoulders AND legs.
It might be good to give your legs a day off here and there so if you train 4-5 times a week switch things up and work on other major muscle groups so your legs don't get fatigued.