In the traditional gym setting, the leg curl is often considered a staple of lower-body strength training. It is an isolation exercise (a single joint movement), knee flexion, used to strengthen the hamstrings.
This exercise is useless because this particular muscle action, isolated knee flexion, lying face down, is never used in everyday activities and sports. So the leg curl is not functional; it has no purpose.
In activities like running, jumping, and skating, for example, the function of the hamstrings is not to flex the knee, but to extend the hip. So, to build functional strength for these activities, you should do hip extension exercises where your foot is in contact with the ground. This is known as a closed chain exercise. An open chain is when your foot is not in contact with the ground, like the leg curl.
One more point is that the hamstrings do not work in isolation; they work together with the glutes (buttocks) in almost all movements. Exercises like the stiff-legged deadlift and especially the single-leg stiff-legged deadlift, which strengthen your glutes and hamstrings, are great functional movements because they work your muscles in ways that you actually use them in your daily activities and sports.
The key to properly performing the stiff-legged deadlift and single-leg stiff-legged deadlift is to focus on bending at the hips while pushing your butt back and maintaining an arch in your lower back. If you remember that the movement comes from the hips and not the lower back, you will perfect your technique and perform this lift correctly.