I'm really curious what benefit it gives, I saw some of the new runs have it with GA first...but why?

The original thought process of doing AS first in a Wizard City% run was for resetting purposes due to the fact that AS (legacy) had the most RNG by far with charred knife and the most difficult bosses. Personally, I never would reset even if my AS wasn't very good so I decided to try GA first instead. I realized that the total time spent walking was decreased; coming out of Firecat Alley, you can mark at Foulgaze's tower for example. It is overall a little bit faster to do GA first which is why many runners have adopted this route.