Why does Linux on x86 use different segments for user processes and the kernel?

Question

So, I know that Linux uses four default segments for an x86 processor (kernel code, kernel data, user code, user data), but they all have the same base and limit (0x00000000

Answer 1

Kernel memory should not be readable from programs running in user space.

Program data is often not executable (DEP, a processor feature, which helps guard against executing an overflowed buffer and other malicious attacks).

It's all about access control - different segments have different rights. That's why accessing the wrong segment will give you a "segmentation fault".