You can fish for them, punish with them, counter fish them. In PvP they create another layer of depths to the footsie and spacing game. They are one of very few ways to reliably stop animation from huge poise builds.
I think the easy to understand reward for learning in PvE and the Depth they add to PvP make backstabs a vital part of the game, as the souls games are primarily driven by the sense of accomplishment one gets for learning and improving in PvE (something backstabs/ripostes contribute to meaningfully) And the longevity of the PvP scene is primarily driven by the complex nature of decision making in combat, a complexity enhanced by the existence of critical attacks.