What Sonic Forces did right with both S-Ranks and Red Star Rings was that they offered more rewards for them.
In previous Sonic games, getting S-Ranks and all the Red Star Rings was more of an accomplishment of talent than anything else. I can't recall if they unlock anything else. But in Forces if you finish a stage you get Avatar stuff. If you get an S-Rank you get better Avatar stuff. If you get Red Star Rings then even better Avatar stuff. It gave me more incentive to replay the stages and try harder even if the levels were piss easy. Sonic Forces has done some things right and actually rewarding for effort is one of them. So I'd say earning Red Star Rings will offer some Sonic Tokens for the player to but things for their Avatars. That or Red Star Rings are a currency to Avatars or something. That could work instead of Sonic Tokens, just Red Star Rings for more exclusive items.
Red Star Rings as a currency, that's what's on one (or more) of Sonic mobile games.
I forgot which one, Sonic Dash 2: Sonic Boom I think.
In Sonic Colors (both OG and Ultimate), getting Red Star Rings unlock a stage too.
A 2D stage with lego-like background, sometimes changing into 3D, called Game Land.
There are 21 stages of them.
7 are available in the beginning and the other 14 had to be cleared by getting about 10 Red Stars each.
The stages are kinda fun for me who prefer 2D Sonic over 3D Sonic, but I already played them in Wii version.