Why the page tables are needed for lowmem?

Hi All i was going through mm initialization code, and saw paging_init() implementation. it has a function map_lowmem(). I was wondering why do we need page table entries for lowmem ?? because all the pages in lowmem can be addressed by kernel logical address and differ only an offset from the physical address. Why cant we simply add that offset to the virtual(logical) address and get the job done. Why waste some memory for creating PTEs for them? can any one clarify? -- With regards, Sandeep Kumar Anantapalli,