What Ashley said is a perfect explanation. However, if your game requires pixel perfect collisions, and you don't want to risk an object "skipping through" parts of your level because the frame rate dropped too much, then that's when you should use a fixed frame rate. Time delta also has an option to prevent skipping like that from happening by slowing down the game if it drops below a certain frame rate. But if you're doing a low res game, i'd say use fixed rate. The tearing is barely noticeable, and it evens adds that old console nostalgia look.
MAJOR DIFFERENCE BETWEEN FIXED RATE AND TIMEDELTA
Timedelta- makes your game have skip lag.
Fixed rate- makes your game have slowdown lag.