First, it has many programmer's tools. Second, it does not allow individual programs to crash the machine. Third, the source code for many UNIX utilities is generally available. It was written for UNIX by Stu Feldman of Bell Laboratories. UNIX provides a veritable arsenal of programmer's tools.
Memory protection prevents one program from affecting the memory owned by another program.
Virtual memory allows a program to access more RAM than actually exists on the machine. The operating system moves running programs between the hard disk and true RAM as needed. Resource tracking means that the programmer doesn't have to free all the resources that she allocates. When a program exits, all of its allocated memory, files, devices, and so on, are deallocated automatically. This means that the operating system can kill a running program and be sure that everything gets cleaned up properly.