Yes but what if one side is so slightly curved that it's invisible to the naked eye? Then your total angles would be 179.99 degrees and it's not a triangle.
I remember using a compass or protractor a few times in geometry class and feeling I could never ever possibly need this because I have no interest in designing things.
A decade later I found myself working for an aircraft manufacturer and needing to verify angles and dimensions for fabricated parts was understandably a critical part of the job.