The obvious "rational" security benefits to users are:
1) Gatekeeper makes it harder to run malware; unsigned executables don't run by default, and signed malware can have its developer keys revoked by Apple.
2) SIP makes it harder for malware to modify system files.
The obvious "rational" business benefit to Apple is that:
3) Gatekeeper makes it harder to sell Mac apps without Apple getting a 30% cut
1) Gatekeeper makes it harder to run malware; unsigned executables don't run by default, and signed malware can have its developer keys revoked by Apple.
2) SIP makes it harder for malware to modify system files.
The obvious "rational" business benefit to Apple is that:
3) Gatekeeper makes it harder to sell Mac apps without Apple getting a 30% cut