Well hopefully you can’t harm your computer with userland programs. Windows is perhaps a bit messy at this, generally, but Unix-like systems have pretty good protections against non-superusers interfering with either the system itself, or other users on the system.
Having drivers run in the kernel and applications run in userland also means unintentional application errors generally won’t crash your entire system. Which is pretty important…
Yeah, security is in layers and userland isn’t automatically “safe”, if that’s what you’re pointing out. So I did mention non-superusers. Separating the kernel from userland applications is also critically important to (try to) prevent non-superusers from accessing APIs and devices which only superusers (or those in particular groups) are able to reach.