Why its required :
If a developer want to make manual changes to SAP sources and SAP Dictionary objects they need a developer key.
How to get developer key ?
1.Go to http://service.sap.com/sscr
2. Now go to section "Service Corner" -> "Register Developer" .
3.in the next screen enter the SAP user ID and then select the installation for which you wan to register the user and press the button "Register".
" Developer GANE01 successfully created for installation 00256910 with Registration Key XCX0CX61824219XXXXX"
Reference:
https://service.sap.com/sap/support/notes/86161