For events/sales on Steam in general, it’s an opportunity for Publishers to showcase their games through events disguised as “sales”.

For the Steam Next Fest in particular, it’s an opportunity for Developers to show their upcoming games, and you get an absurd amount of demos available to play, for free!

For me, the Next Fest is a nice way to find interesting games, see what the indie scene has to offer, and have access to free games I can play in short sessions. Every edition, I download around 50 demos, and try to play them throughout the week (and the following week because some are still available after the event ends)