While the contract is open-source, if you were to copy the source and deploy it on the ethereum network yourself, it would have a different address and you would have to convince people to use your contract over the creator's. For that you would either have to offer lower fees, build a better brand value, add more features or build a better advertising campaign.
Keep in mind that open source doesn't always mean free to use. If the contract is not published with a license, maybe that gives you permission to copy or deploy it