Something I Do Not Understand About Trainers, Fitness Instructors, Coaches, Etc

Just because I can do something, doesn’t mean I should do that something + more or harder or faster. Sometimes it does, but not every time. Just because I can hold 1:20s at swimming doesn’t mean I should, nor does it necessarily mean I should try to hold 1:15s and not make it through the set. Just because I could do plank push-ups at TRX today, didn’t mean they were easy or that I should do something harder. I actually could only do a set amount of them before my arms gave out — but I guess I looked stoic while doing it, so the instructor kept trying to get me to do harder stuff until I couldn’t. Why the insistence on pushing until failure? That’s fine sometimes (sometimes we need that), but sometimes doesn’t it make more sense to do the hard thing that you can do well, instead of the harder thing that you can’t quite do poorly.