A customer hires you and you then perform a service. Once that transaction is completed, so is the contract between the two parties. If the customer ever has a new project, they start at square one when it comes to finding someone to hire.
Alternatively, a client already has an invested trust into you and what it is that you do. They know about the services you provide and what separates you from your competition.
So you have two choices.
You can spend your time chasing new customer after new customer.
Or you can invest into the relationships that turn a customer into a client.