FrogReaver
The most respectful and polite poster ever
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The reason why you might need a Tough feat to represent a tougher-than-average character is because there aren't a lot of ways to represent toughness, given the simplicity of the model. The primary metric for toughness is just your Con score, but a lot of people have a very high Con score. If everyone in the party has Con 20, which is a thing that happens more than you might imagine, then everyone in the party is objectively very tough and nobody is noticeably more tough than anybody else (although the fighter is more skilled than the wizard, if they're of equal level, and so has more HP). If you want to play a noticeably-tougher character in a party where everyone has high Con, the Tough feat gives you a way of doing so. (So does the Hill Dwarf, of course, but racial options also come with a lot of extra baggage that might not fit your character concept.)
I skipped right to the meat of your post. I hope you don't mind.
On a side note: Toughness would actually be a measure of hit die size and constitution score.
1. Why do other characters toughness have anything to do with your characters toughness?
2. Why is it an issue for everyone in the party to all be very tough?
I guess the only legitimate complaint I'm hearing is how does one progress in toughness after you have reached the 20 stat cap on toughness. In that case feats aren't the answer. Instead the answer is either uncapped ability scores OR not having ASI's at all.