In the traditional gym setting, the leg curl is often considered a staple of lower body strength training. It’s an isolation exercise (a single joint movement), knee flexion, that’s used for strengthening the hamstrings.
I believe this exercise is useless because you never use this particular muscle action, isolated knee flexion, lying on your stomach, in everyday activities and sports. So the leg curl is non-functional; there’s no purpose to it.
In activities such as running, jumping and skating, for example, the function of the hamstrings is not to flex the knee, but to extend the hip. So to develop functional strength for these activities, you want to do hip extension exercises where your foot is in contact with the ground. This is referred to as a closed-chain exercise. An open-chain is when your foot is not in contact with the ground, such as the leg curl.
One more point is that the hamstrings don’t work in isolation; they work together with the glutes (buttocks) in almost all movements. So exercises such as the Stiff-Leg Deadlift and especially the One-Leg Stiff-Leg Deadlift, which strengthen the glutes and hamstrings, are great functional movements because they work the muscles in ways that you actually use them in your everyday activities and sports.