I believe that tempos are particularly useful when you want to accurately measure your progress.
Example: Say that you have been benching 250#'s for 4 reps @ a (301) tempo. You embark upon a strength training program in hopes of getting your bench press poundage?s up for the given amount of reps.
Now the next week you go in and put 255 on the bar and get 4 reps, but your tempo is(X0X). Did you really get any stronger?
Also, tempo will help you gage what works for different goals. Say you embark on hypertrophy program and plan use the old 8 to 12 rep range. However, you perform your reps at about a (101) tempo which would equal to way less TUT than what is considered optimal for hypertrophy, therefore you muscle mass gains are minimal. This may lead you to believe that the program was crap.
These are simplistic answers at best, but I hope it helps.