The point being that, when you create MetaNet transactions you need to spend from a specific authorized key. Often when creating a UTXO, you do not yet know which keys you wish to spend from in the future.
By using RPuzzles, you can create UTXOs (with the same unlocking script layout as a regular P2PKH unlocking script) which are spendable by any key, given you provide a secret (k) which deterministically creates value R in your signature.
The point being that, when you create MetaNet transactions you need to spend from a specific authorized key. Often when creating a UTXO, you do not yet know which keys you wish to spend from in the future.
By using RPuzzles, you can create UTXOs (with the same unlocking script layout as a regular P2PKH unlocking script) which are spendable by any key, given you provide a secret (k) which deterministically creates value R in your signature.