mmap_min_addr info

quoting the debian wiki for mmap_min_add. "Allowing processes to map low values increases the security implications of a class of defects known as "kernel NULL pointer dereference" defects... The downside to preventing applications from mmap'ing low virtual memory addresses is that certain applications depend on this functionality. dosemu, qemu and wine are three such applications that exist in Debian" I don't understand the use of the mmap_min_add. What's the point of preventing applications from mapping to low memory values. Also why would applications mentioned here would need to map to low memory addresses? A little descriptive explanation would be very helpful!