I think like with any other standalone emulator compared to RetroArch, the main benefit would be the user interface and the entire configuration setup. Ares is a more traditional application with less functionality and narrow focus. That makes it simpler and more native to use on your operating system. Also you don't need to learn and understand how RetroArch is setup, with all its configuration, because how many features it supports and how generic it is.

Note: I'm a huge RetroArch fan, its not dunking on it or anything like that.

Also some people don't like that RetroArch itself does not have emulator cores, but rather use other cores (which is the entire point of it and why I'm loving it). Ares is completely its own thing (but a fork of higan, which was based on Bsnes, RIP Near). And because its much simpler, people might be understanding its source code better.

Also I don't use and don't know what Ares actually supports and can do. In example I have a separate and standalone Mesen, just because of its excellent debugging features (when I experimented with romhacking).