Depends how serious you want to be about your training.

On the bike, I like to look at the tracks in retrospect to monitor my fitness, see whether "that hill" was really as difficult as it felt, whether I was having trouble with the heat, etc. I don't use it while riding at all.

Running, I do use it for training, to make sure I'm not going harder than I want to during recovery intervals between repeats, and on my long runs.