The primary reason we use this model is for predictability. In the past, we’ve priced work both per-project and per-hour and have had success and challenges with both models.
After 10+ of working with teams of all sizes, we’ve found that the “subscription” (most similar in sprit to a retainer) model works best for both us and the client. We know how many clients we will have per sprint and benefit from predictable revenue to sustain our team of 11 people. Clients benefit from peace of mind knowing that they can get continuous high-quality design work without trading estimates and invoices (if it’s a quoted project), highly variable billable hours (if it’s an hourly engagement), or paying a full time salary for a top quality senior designer (>$100,000 + benefits + recruiting).