It's just another playstyle. Some maps are more fun with full share, some are more fun without. Imagine gap with fullshare? Yikes. Imagine setons without? Nearly as bad.
Imo, full share's goal is to make the feeling of a game being a waste of time not happen. Imagine you're playing setons, you're doing fantastic on your side, but your teammate is an idiot and suicides somehow. No matter how well you play, your chances of coming back from completely losing one of the lanes is not that high. You either have to start going for snipes on your own, or attempt to win your lane real quick. And if that was possible why didn't you do that already?
Ofc you can reply and be like, well that's part of the game! Yea, no shit. But a lot of people obviously prefer still having a chance to win if someone on their team isn't doing so great. It makes certain maps more enjoyable and allows for different strategies.
For example, I love playing seton's mid. One of my favorite positions to play in FAF. That said, I'd hazard 70% of my games I'm presented with a situation where I can kill the enemy mid ACU. But then what? Do I have enough to keep pushing? Is my eco good enough to counter a now double eco'd player? Should I let the enemy mid live and just eco myself? Would I benefit the team more by switching to navy or rushing an experimental instead of sending a ton of mass to potentially donate down mid?
If you didn't use full share on setons, mid would go from a defensive/supportive position to probably the most important position because if you die, it's gonna be a pain in the ass for your team to stop what's coming. Navies will have to pause, air will have to switch to bombers/gunships. And at that point, now every lane is behind.
I'm not saying setons without fullshare can't be fun. It can be. But at least for me, it's more fun and the games vary a lot more with fullshare on.