Sometimes, a recording or a vocal performance sounds great but it gets ruined by the harsh S and T sounds. This usually happens in vocals since a lot of times they increase the air, the breathiness, and brightness of the vocal.
De-esser is such a powerful tool in cleaning up the vocals. You really don’t need to buy one since more often than not, your DAW was shipped with one for free. There are default presets on the software and you could choose between applying it to the high frequencies or the whole track.
Read more about this here: https://www.recordingrevolution.com/how-to-clean-up-harsh-vocals-with-a-de-esser/