Why is it required to create Appian User Accounts in dev environment?

I'm not really sure where this question is coming from, but Appian User Accounts are nominally required to do anything at all within Appian, from development work to administration and o&m to the front-end seen by basic users (which in a dev environment will often be test accounts representing specific user roles rather than real people).

If you mean "Why do we need Basic accounts in DEV" (as opposed to just System Administrator accounts) there two main reasons that I can think of:

• it's best practice to Unit test your code with Basic User accounts (as if the code was being exercised by real users). You'll then spot where you've not configured the security properly, rather than have this discovered in your TEST environment
• You can in fact segregate Developers from each other so that one set can only see and work on one Application, and another can only see and work on a different Application. You can do this by creating Basic User accounts but then giving them Designer rights, and then adding them to the Group that controls visibility/access to a given Application.