Logically and functionally bucketing and partitioning are quite similar - both provide mechanism to segregate and separate the table's data based on its content. Thanks to that significant further optimisations like [partition] PRUNING or [bucket] MAP JOIN are possible.
The difference seems to be imposed by design where the PARTITIONing is open/explicit while BUCKETing is discrete/implicit.
Partitioning seems to be very common if not a standard feature in all current RDBMS while BUCKETING seems to be HIVE specific only.
In a way BUCKETING could be also called by "hashing" or simply "IMPLICIT PARTITIONING".
Regardless of the fact that these two are recognised as two separate features available in Hive there should be nothing to prevent leveraging same existing query/join optimisations across the two.
Enable partition PRUNING equivalent optimisation for queries on BUCKETED tables
Simplest example is for queries like:
"SELECT … FROM x WHERE colA=123123"
to read only the relevant bucket file rather than all file-buckets that belong to a table.