Generally, they offer a giant infrastruture that no other game selling platform offers:
- cloud saving
- community hub for sharing user content
- inbuilt forum
- mod database
- better shop visibility through intelligent algorithms
- community events like NEXT fest
overall, you get what you pay for.
Ignoring the shady practices of Brave Software, this doesn't really solve the problem. Sites will still use way too much scripting to be flashy, and that will continue to be a problem for everyone, because some of these sites willbe needed for some and will require all scripts to function properly.
What might help more in the long run is complain to the site owners that their site, despite you having an up to date browser, does not work on your phone. Sure, some of those complaints will fall on deaf ears, but even some changing means progress.