A subscription payment gateway helps collect recurring charges. Freelancer accounting software helps a self-employed business keep track of clients, invoices, costs, and financial records. A monthly retainer can involve both, but collecting the retainer does not explain the project's expenses, unbilled work, or overall profitability.
The practical difference is between collecting money repeatedly and managing the business behind those payments. Some accounting products already connect automatic payment services to repeating invoices. A separate subscription setup becomes useful when the freelancer's selling model needs capabilities beyond that existing connection.