This is a common protocol feature. For example a .WAV file starts with WAVE - the letters RIFF can also appear. Using ASCII letters makes it easy to see that you have the right kind of format just by inspection (if you deal with ASCII often). Often binary dump views will display the ASCII if it is within normal "printable" character range as added info. This is also helpful to make sure that you are at a "proper beginning". Decoding the thing requires being certain that you are at a valid byte(0) because all field definitions are a delta to that byte's "address". So this is a cheap (very little bandwidth), "sanity check". This is also often seen with communication links and can assist with resynchronization when some data "goes missing".