why do we need a separate environment for the developers and testers

This is to prevent developers and testers from stepping on each other’s shoes. Developers are continuously making changes for fixing defects or creating new enhancements. This may cause side effects which include but not limited to:
⦁ Unintentional adverse changes e.g. introduced new bug while fixing a different one or accidentally wiping out the database
⦁ Could slow the application/system by turning on debugging
⦁ Could bring down the application/system by applying/building changes while testing is in progress