As already pointed out, some use schema as a namespace feature.

Also within a very large monolithic app you might still want some separation of access given to different modules, so a nasty bug that allows injection or inspection attacks has a more limited scope for causing DoS or exfiltration. You can control access on a per-table basis or even per column in some DBs, but the schema can be a convenient place to configure that over a larger surface in one go.

How useful any of this seems to you is going to be a subjective thing.