Is there any point with a combined index on all columns in a table?
The index is a sorted structure - if you need only a sufficiently small portion of the data in the table, it could be fetched from the index more efficiently.

This needs a few prerequisites, though:


PostgreSQL version 9.2 or newer, as index-only scans appeared in this version
the index supports the query (the order of the columns of the index decides this)


Also worth noting that even index-only scans must visit the underlying table (to be precise, its visibility map, and in cases the heap itself) to figure out if the rows to be returned are still visible to the current transaction.  This has to be also considered when determining what 'sufficiently small' means above.