I've been wondering whats the benefits of a Stable-coin?
It's something that can be used as a hedge against the volatility of bitcoin and the cryptocurrency markets in general.
I haven't seen any places where you can pay with lets say Tether - so what makes people "invest" in this coin?
From my point of view the only rational reason to invest in Tether - is to trade for other Cryptocurencies. But then again i might've missed something

You guessed it right. It's mostly only for trading; not as an "investment"(obviously, they're just going to be $1) as no one "invests" in USD anyway

. Especially if you're anticipating a crash in the price of a certain cryptocurrency; you would sell that coin/token for certain stablecoins to sort of "protect" your investments, so you could use those stablecoins to buy back lower, or to buy other cryptocurrencies.