It allows for arbitrary data to be included with a transaction that is labelled outside of the uxto information. Its a way to inscribe information permanently into the chain of blocks. Some have used it to make sure bible verses, or certain events. Its been abused by linking to jpegs ect

It is prunable data so your node may choose to not include op return in the utxo management. So theoretically it limits the boost of the chain of blocks data set.

* I just researched this