Partial indexes allow you to do stuff like keep session tokens for some longer time to facilitate logging of attempted use of expired credentials, without significantly hurting your authentication-validation performance. In most cases you will not hit the disk, as the partial index is in memory.

In general, you can keep selective indexes in memory, possibly even with supplementary columns to eliminate disk access for the actual data you're trying to fetch by the key the index is based on.

Even Sqlite has partial index support...