Sorry, but I don't know how to write this without a severely pedantic tone. Please, ignore the tone. We aren't all the same height but have an average height. I'm sure that someone (not me) can tell you the average age of 52 year old males in Missouri etc. That would be a starting point, just as 220-age is. It is a rule-of-thumb or spleen or something. It is a good starting point and we can all improve on it if we wish.
I am sure that resting HR is interesting just because the Dr. always checks it. Athletes like to brag about their minimum HR which is ~wake-up time. I guess that it is a useful health indicator.