If it's anything like onshape (the only browser based CAD I know of), browser based means: everyone uses the same version of the software, you don't have to wrestle your it department for installation rights, you can login and work on your designs from anywhere with any average laptop, there are no files (houseV3finalfinal), probably proper baked in version control with branching and merging... everything software developers take for granted that we poor CAD designers only can dream of.
A browser based collaborative CAD tool appears to be a great idea. Rendering is now longer the bottleneck in computation and Nurbs data is actually lightweight. Computation heavy tasks like the geometry engine could be done locally. However developing a geometry engine or nurbs kernel is quite a challenge- harder than any game engine.