I agree with what Hillary said. The fee also goes towards the payment processing, which is a key reason why I signed up with Rover 2 years ago. They handle all the payments, so I don't need a merchant account or a credit card processing setup, and if there are any CC issues, Rover can deal with them.
You never have to pay for anything (maybe the initial background check?). You can purchase the protection package for $50~ but that is not required.
The key to getting clients is to have a rocking profile. Explain why owners should choose you - why you love dogs, your experience, etc. Give details of the services you will provide - I do boarding so I tell them what the dog(s) and I will be doing together. If you are boarding or doing day care, include photos of your home where the dogs will be staying. Owners love to see where their baby will be living. State up front if you have any limitations or special circumstances, such as you own cats/dogs, have children, etc.