Is there a practical reason for orphaned blocks being stored ?
In case of a Blockchain reorganisation, perhaps.
Once a block turns out to be orphaned the valid transactions get added to the mempool again.
But what is the actual reason to further store this block if the information it cointains is no longer needed?
There's no way to tell it's "no longer needed" though.
There's no central arbiter that decides whether a block is needed/valid or not.
Nodes take the longest chain with the highest cumulative POW as the canonical blockchain.
The canonical blockchain could change if nodes receive another block that follows consensus rules but has a higher difficulty than the one they possess, even though the probability of a blockchain reorg 6 blocks deep is small, it's still theoretically possible.
For example the
March 2013 reorg was ~21 blocks deep.