Index creation can take a very long time for a large database, think hours or god forbid days. Restarting from scratch for an index that takes 18 hours to build is very painful.. doubly so back when concurrent index creation didn't exist.
REINDEX INDEX CONCURRENTLY
I'm honestly not sure if this is saving anything except re-defining (i.e. the literal "create index" statement), but they seem definitely not 100% useless despite being invalid. Maybe 99% useless, but "drop it in the background" doesn't seem particularly better either - leaving intermediate state for an admin to tackle by hand seems reasonable.
And how do you clean up after an aborted index build?
[1]: https://www.postgresql.org/docs/current/sql-reindex.html