If you just host for yourself, you don’t gain that much by using Seerr, besides having a nicer UI and you have more search filters compared to Sonarr and Radarr.

However, if you have multiple users, you benefit a lot of it. Users, which have individual user accounts, can request media. Depending on the configuration, those requests have to be accepted manually, which gives you a way to still be in control of what ends up on your server. The user then gets notified about what has happened and if the media was downloaded.