> Then I can.. sell it for $100? What’s the point?
You can trade your stablecoins against other cryptocurrencies that they are paired against on exchanges or you can loan your stablecoins to a platform for a profit so they can lend it out to others for a profit. Banks do the same thing. When you deposit money into a bank account you are essentially lending the bank money. We call it a deposit but it's not like the bank is actually holding 100% of everyone's deposits.
> If they promise returns where are they supposed to come from? If my coins are pegged to USD how do they appreciate? Or is the idea they just give me more for holding?
A low risk way of generating a return would be the platform lends out the coins you have locked as a secured loan for a profit.
The high risk way of generating a return would be the platform you have your stable coins locked up on finds another lending platform generating a higher return than they are promising and they invest your locked up funds there.
> But if the coins are pegged where do they get the money for the new coins they’re giving me?
They are created out of thin air. Basically they have to incentivise people to use their coins while keeping enough funds on hand to mop up any excess liquidity on exchanges when the price of their stable coin breaks peg. Governments do the same thing with national currencies. They incentivise you to use national currency by naming it legal tender and enacting capital controls as we have seen recently in Russia, Turkey and Lebanon.
If they don’t promise returns why would anyone invest in a stablecoin?
In some countries trading from crypto to crypto is not a taxable event unlike trading crypto to dollars. Stablecoins were embraced by exchanges too so it really is a lot easier to trade crypto for crypto than it is to trade currency for crypto or vice versa. Essentially people don't invest in stable coins for the most part. They use them because they have a greater utility than actual dollars.
That said buyer beware! Cryptocurrency is a largely unregulated space and regulations exist for a reason!