Comment by tranq_cassowary

Note that the user profiles aren't the basis of sandboxing. The bank app is still sandboxed, and GPlay as well if you only use a single profile. Profiles have use cases of course, they offer separate app data, user data and VPN configurations. They also have a separate list of apps marked as installed, a list apps can see in a profile. IPC also can't take place cross profile, which is useful to prevent communication between apps based on mutual consent. Also nice is that they offer separate encryption keys and can be put at rest manually. In the past they also had the advantage of having a separate clipboard but this isn't an advantage anymore given that GrapheneOS recently shipped within-profile clipboard control.

If you dont need any of the above its perfectly fine to run the banking app on your single, main profile.