Why both ioremap() and kmap() are needed?

In case of memory mapped IO: both memory mapped IO and RAM are in same CPU physical address space.
When we map any higmem memory we use kmap() and when we need to map memory mapped IO we use ioremap().
So my question is why we need to separate functions to get kernel virtual address, since both (memory mapped IO and RAM) are in the same physical address space?
Why can't we have single function, as both are creating virtual address for some CPU physical address.