Why are certain games locked at a specific framerate and are there any benefits to it?

Been looking at people make recomps and decomps of games and showing how for example, Metal Gear Solid 3 on 3DS is locked at 20 frames and can't go above it. Anyone know what is that?

Is there any benefit that I'm missing as to why it's a good idea to lock the frame rate in games? Cuz I see a lot of people make complaints when playing them normally or on an emulator but then when a Recomp comes out for PC, one of the main things mentioned is unlocked frames so it can be at least 60fps or higher