You call ptrace and ask to read another process's memory. The kernel has to turn the requested VA in that address space into a physical page (walking those page

You call ptrace and ask to read another process's memory. The kernel has to turn the requested VA in that address space into a physical page (walking those page tables, not yours) then copy the memory (from some VA in kernel space). This is quicker if you can turn a PA into a VA by simply adding an offset.