Lindsay is exactly right - the "pee in the cup" is a one shot look at kidney function - it's a good estimation, but not as precise as looking at a 24 hour sample. Anytime you have any kidney problems, then a 24 hour urine is in order. I had to do one last year to check if I was getting rid of too much calcium (I wasn't). My bone density test was lower than previous, despite taking meds to prevent bone loss - so they needed the 24 hour urine to check for calcium loss. It turned out to be most likely measurement error, rather than actual increased bone loss.
The hard part is being home to pee or taking your jug with you to where ever you are going (not an easy thing to do). The urine has to be refridgerated, so a big old jug of pee in your fridge (or someone else's fridge) is not very appealing! LOL! Gary and I always do it over the weekend to avoid those situations.
