ACCESS KEY - First of all its provided by SAP. You can get the key from http://service.sap.com -> SAP SUPPORT PORTAL -> Keys & Requests -> SCCR Keys -> Registration. Here u can register a DEVELOPER or an OBJECT, by this time SAP will give the ACCESS KEYS.
**Note : To access http://service.sap.com u should have S number login or like that.
Only after obtaining that ACCESS KEY, SAP will allow the DEVELOPER to create some objects or change or modify Delivered objects.
Why ACCESS KEY - For creating or accessing any object some one should be responsible. Suppose, any issue arises with an object then can easily track the details who has created or modified.