They can be used for any logical scenario that you need. Such as

• An extra QA/UAT envurionment if you're running parallel streams of work and want to deploy packages for testing at the same time
• A pure staging box for experimenting with content creation for clients

Horses for Courses really for one client we use as follows 

• ADE1 - Sandbox - Deploy any code change, accepts feature branches, to test or demo functionality. 
• ADE2 - Development - Deploy only master / main branch in a CI/CD process for end to end developer testing 
• Integration - QA / TEST - Promoted codebase from ADE2 handover and QA 
• Pre-Production - UAT / Staging - As above but for handover to client and them doing acceptance testing. 
• Production

For another client its similar to the above from Integration / Prep and Prod aspects although we have an additional dev environment purely as another integration env as working on multiple workstreams. 

ADEs, or Application Development Environments, provide developers with a comprehensive suite of tools and resources to create and deploy applications. The use of an ADE can offer many benefits, including increased productivity, streamlined development processes, and improved collaboration. Here are some of the reasons why developers choose to use ADEs:

Standardization: ADEs provide a standardized environment for development, testing, and deployment. This helps ensure consistency across all stages of the development cycle, which can reduce errors and improve the quality of the final product.

Increased Productivity: ADEs typically include a wide range of tools and resources designed to increase developer productivity. This can include code editors, debugging tools, version control systems, and more.

Collaboration: ADEs can facilitate collaboration among developers by providing a centralized platform for sharing code, documentation, and other resources. This can help teams work together more efficiently and avoid communication breakdowns.

Simplicity: ADEs can simplify the development process by providing an all-in-one solution for development, testing, and deployment. This can reduce the need for developers to switch between different tools and platforms, which can save time and reduce errors.

Security: ADEs can provide a secure environment for development by implementing security protocols and policies to prevent unauthorized access to code and other resources. This can help protect against data breaches and other security risks.

To conclude, ADEs offer many benefits to developers, including increased productivity, streamlined development processes, and improved collaboration. By providing a standardized and secure environment for development, testing, and deployment, ADEs can help developers create better applications faster and with fewer errors.