1) Nowadays, there is nothing to draw new players about Gmod. It's not got pretty graphics, limited ability within Source 1. But S&box (from what I hear) is supposed to be a lot better for developers and allow a more expansive variety of Gamemodes. 2) A lot of gamemodes could use a rework and S&box is the best place to do it. 3) A new game release gives facepunch a reason to advertise allowing for the game to get more attention the Garry's Mod has received ever since the peak of it's popularity a few years ago.
Negatives for S&box
1) Developers may be scared off from switching from GLua to C# (C# superior fight me) 2) Servers may not transfer due to there being a lack of players in the beginning. Possible snowballing into a lack of addons, and a lack of content. 3) I hate the new Playermodels so far, hopefully they allow us to mount Half Life Alyx later. Or maybe mount Gmod to use those models. But like hell we'll get by with the current PM's imo.