Most people use it to convert their alts during bleeding season. They don't have all that money on hand to be provided and most people already realize that. Use it as a back up to save yourself from crash but don't hold in for too long. that would be my suggestion. By now, there are many other alternatives to tether if one chooses to use. Risk is similar.
usdt is used for trading at cryptoexchanges. Amount of trading in usdt corresponds to amount of trading in btc and eth.
https://coinmarketcap.com/ - cryptocurrencies tab
compare Volume (24 hour) for ( btc + eth ) and usdt