Another reason could be that the person wants bigger real screen estate when playing android games with the use of physical keywords and mouse. A developer who wants to test his apps before releasing the app to the public would use an Android emulator of their choice to test the app.
People, who never use an android emulator in a windows 10, 8 or 7 PC will always have this question in their mind that why would you need to download emulator if you have an Android smartphone which can simply run android apps and games without any other external tool? Well, it could be any reason for someone to run Android apps on a Windows/Mac PC. Your computer becomes a dynamic device which can run other apps and software which are only compatible with another operating system like Android. Android Emulators are one of the most amazing things for Windows PC and Mac users.