These mechanisms allow a program to drastically limit the amount of damage it could do if it's subverted by bad/malicious inputs (or something goes wrong becaus

These mechanisms allow a program to drastically limit the amount of damage it could do if it's subverted by bad/malicious inputs (or something goes wrong because the program itself has bugs..) Imagine a program that connects to a host over the Internet. And someone malicious is in control of that host. If that malicious host manages to take control of the connecting program by exploiting a vulnerability in it, it still doesn't gain full access to your local machine, only to the limited subset of functionality that the connecting program has pledged. :)