It's not about HPS, it shouldn't be at least, but that is the easiest and most common tool used when assessing healing, though that doesn't mean it's the best.
I don't mean to give the impession of HPS being the only thing that matters (did I?). You shouldn't need to check the meters if a fight goes smoothly and there were little to no deaths and Ardent Defender didn't proc etc. However, if there are healing difficulties, it's nice to check a healing meter and wonder why player X is doing a third of the healing as player Z.