They're not perfectly stable. The issuer guarantees you something about them: maybe that you can redeem it for $0.95 today (perhaps phrased as $1 minus fees), o

They're not perfectly stable. The issuer guarantees you something about them: maybe that you can redeem it for $0.95 today (perhaps phrased as $1 minus fees), or $1.0 in a year. So the price will fluctuate a little below $1, based on interest rates, creditworthiness of the issuer, and simple supply and demand. This is exactly like a bond. The trade is to buy the stablecoin at $0.96 and wait for a chance to sell it at $0.995. Your investment thesis is that someone else is selling at $0.96 not because the issuer is going bust, but because they need the funds for something else.