You can do unique things like offer a trial period without syncing (database wholly in browser), which allows for ridiculously quick onboarding. And then when the user signs up for a paid plan, you just hook up CouchDB and the data syncs right on over.
The sync mechanism works so damn well. It's really cool how easy it is to implement a Google Docs-like sync mechanism with conflict resolution baked in.
PouchDB does have rather expensive adapters that use memory or LocalStorage. Almost twice the size as the base package (minified).
I really liked it, but the typical pattern of one-db-per-user with internal replication on the server was a bit difficult plan/orchestrate for shared data. I'm patiently waiting for the couchdb PR for document level access control.
These days I use supabase with row level security and a Vue-wrapper that can cache queries locally and update the result as the network request finishes. Works as good as pouch + couch (but naturally comes up short for queries that rely on Date.now())
I'm really surprised more companies don't pick this stack.