The only thing that I need to know the accurate distance is to train a specific event, e.g. mile run, 5k, or 10k.
If I train to run a mile within four minutes, all I need to know is to run 60s for each 400m on track.
It’s also nice when you want to do any sort of interval training without actually going to a track.
Having accurate overall distances is less important, but is useful if you’re training for a specific distance and want to run some time trials without using a track or measured course.
I tend to plan my pacing for equal effort, so each lap pace is adjusted for elevation such that in the end the overall pace is just slightly faster than what is needed for goal pace.
Needless to say, I use my Garmin Forerunner 945 for this.
If anyone is running the Seattle Marathon, I'll be there pacing :)
If you're tracking incremental progress than such variability makes the data less useful. But that's why I prefer to use natural landmarks and a stopwatch - the smart watches I've used so far are off to the point where I don't trust them.