As far as I can tell, there's no purpose behind the hashing, besides block generation. Miners maintain the shared ledger by forging blocks which contain transactions.
The reason it is hard to do, and increases in difficulty as more and more people mine is that it protects the transaction record. If the difficulty keeps up with the current computing power it makes it effectively impossible to forge transactions. Typically 6 blocks or about an hour is when transactions are considered rock solid. No one could have enough computing power to change a transaction 1 hour in the past.