This predates Android by years. Special "phone number" codes have been used to control firmware since the very first compute went into a phone. The reason is fairly clear: in the early devices, dialing a number was the only UI metaphor available. USSD itself is actually a standard, such as it is: http://en.wikipedia.org/wiki/Unstructured_Supplementary_Serv...

Now, of course, it's just a bit of legacy nonsense that gets left enabled simply because it's part of an existing workflow and serves mostly as a hidden gotcha for people doing security analysis.