You’d use only one.
Time is somewhat easier for the human brain to work with. If I give you a long spline of unknown length and ask you to point to a spot at 42% of the spline, you’ll have no issues. When you work with Time, you can work in a unitless fashion. Or you can work with actual Time, if you wanted to move to a location that is 3.5 seconds away, you can:
And there’s also Constant Velocity toggle for Time. Wouldn’t work well for a rollercoaster cart but could work well for a camera rig that needs to move in a very specific fashion.
If I give you a spline that is 7380 uus and ask you to point at the point at 3000 Distance, now you need to apply some mental gymnastics or worse, do math. 
Both have their uses - kind of depends on how we approach things. You can use both simultaneously, but you do not need to.