This process is called segment swapping. If there isn't room for a program to load completely, segments of program already in memory may be swapped to disk to make room for the new program. The result is the amount of memory available is no longer equivalent to the amount of physical memory. Instead of directly accessing physical devices like parallel ports and video screens, programs are presented with virtual ports and screens. Key roadblocks of DOS are eliminated.
Programs can use more memory, and they can no longer interfere with each other.
But not only are virtual memory and device management the basis for multitasking, they are the basis for multiuser operation.