Right, you certainly don't need to track yourself for 10 years to figure out how long a shower takes!
A rough analogy: you can figure out from your HP bar how much damage you take from each hit, and it's probably worth doing so if you're going to be fighting these same baddies forever and have no other way of knowing, but the HP bar also serves other important purposes related to resource management: it gives you a sense of how you're doing overall, and it constantly reminds you that HP management is even worth caring about.
A rough analogy: you can figure out from your HP bar how much damage you take from each hit, and it's probably worth doing so if you're going to be fighting these same baddies forever and have no other way of knowing, but the HP bar also serves other important purposes related to resource management: it gives you a sense of how you're doing overall, and it constantly reminds you that HP management is even worth caring about.