It's the most basic form of an anti-malware, long before a traditional antimalware works.
Consider that an OS is a software ecosystem where multiple applications developed by multiple parties interact. Exactly like a big bazar where multiple merchants offer services and goods to the same customer. They must share the common space. And some may be evil under any extent.
Sharing of resources
Isolation between kernel mode and userland mode allows OS to prevent an application from stealing too many resources, making it safely crash when it comes to requesting too much memory or disk space. In this scope, OS also prevents an application to take control of disk by overwriting data related to another application.
Applications may contain sensitive data. Your OS stores your user password, or at least its encrypted form, in memory. If OS did not isolate its own memory from the app memory, any app can tinker with your user password.
Not to mention with your Bitcoin wallet...
Do you really want an application crashing to reset your entire device?
Think for a second: you are on a Skype work meeting and your favourite messenger application crashes because of a bug. OS makes the IM crash, and notifies the event, but your computer/tablet will not reboot so you won't have to explain your boss why your video was interrupted.
This third case covers unintentional faults.