Where this usually helps
Prototype jobs commonly include prototype samples, fit checks, and early product validation. Some are mainly for presentation, while others need enough dimensional accuracy to test whether the idea works in real life.
The real benefit is iteration. The first print helps answer what to change next, whether that is size, mounting, tolerance, visual appearance, or how the part behaves when handled.
Custom enquiries usually involve at least one non-standard condition: exact dimensions, special wording, file adjustments, fit requirements, or a small-batch need that should not be pushed through a generic online store flow.