A lot of agencies sell the ability to easily find a new developer for your project if one leaves.

And a lot of agencies actively replace developers on projects frequently, as they optimise their teams across all their projects

Despite agencies rationalising this as reasonable in various ways, it's not ideal

When any new developer starts on a project they'll need a day to set everything up and do basic familiarisation. Longer if the project setup is not well documented and automated. In addition to the technical side, they need to understand the team's workflows

As they start work, they'll start to learn the structure of the application is and how it's configured. The time this can take is variable but it'll be anywhere from days to weeks, again depending on the complexity of the project.

The quality of the codebase, the amount of automation, the documentation, and other factors all influence the time that the above will take.

If you're hiring an agency, you should ask them how they handle developer onboarding both at the start and thing the project. Ask them if they make a habit of rotating developers on projects. Ask them about levels of documentation (and ask for examples).

Finding an agency that handles this well has a direct impact on your ROI.