Back in the day, on Bitcoin exchanges, you could have Bitcoin price difference of say $200 USD on a $1,000 Bitcoin price. So you could sell on one exchange and buy on the other and make 20% Fiat each time. This, importantly, left you with an unchanged Bitcoin amount, unless you bought more BTC. That is called arbitrage.
The problem was repeating the arbitrage, getting cash between Bitcoin exchange platforms. Sending Bitcoin could take 30 mins. Sending Fiat could take upto a day or more if you got close to a weekend.
Tether previously Realcoin fixed that problem, holding the same amount or increasing Bitcoin amounts, while being being able to make 5 to 25% every time until the prices on the exchanges became the same.
Very profitable and made possible by Tether.
It's never been scammy at all, they with Simon Dixon helped to rectify the Bitfinex hack. Where I would have lost about $50k, if I remember right, without Tether helping out. Clearly not scammers.