There are many possibilites, the Autocad keygen also needs some memory patching, doesn't it?
IIRC you start it with the Autocad Registration dialog open and have to click mem patch before you paste the activation code.
Since normal applications run in user context you have to start it as admin to allow it to modify memory space of other running apps.
Other keygens/patches/cracks/licgens/loaders might need admin rights e.g. to...
- ...access hardware details that are needed to build up ids or request codes
- ...open binary handles to patch other files
- ...modify memory or code of the target (e.g. to circumvent certain checks, replace the internal hardware id, re-whitelist some blacklisted serials, whatever...)
- ...write to protected registry entries
- ...write to protected system files (e.g. hosts file)
- ...unpack themselves if they use some custom packers to protect the cracker's code
- ...many more I have no clue of
Not everything needs admin access just to infect your PC