Now it seems you want to know why there is kernal space and user space
The user space, which is a set of locations where normal user processes run (i.e everything other than the kernel). The role of the kernel is to manage applications running in this space from messing with each other, and the machine, and
The kernel space, which is the location where the code of the kernel is stored, and executes under.
Now assume you have a program which calls few system calls which is part of kernel hence kernel space. If we don't have such mechanism then one has to write.