It is the reflector (in another word: mirroring) that makes it impossible to encrypt any letter into itself. I am not sure whether the inventor of Enigma had intentionally made this effect. If he had, the reflector is really a clever way to guarantee this point. But if he hadn't, the only purpose of he introducing the reflector seems to be
making the processes of ciphering and deciphering symmetric on the same keyboard on the same machine, which is an aesthetic of this ciphering machine.
Note also, please: the mechanism of reflector contributes nothing to making the number of combinations larger and thereby making the ciphering more complicated.