This allows the QRCode decoder to find which mode we are using.
There is also a special mode, ECI, which consists in mixing different modes.
"HTTPS://FAST-QR.COM/blog/mode_indicator" could be split in two encodings: the first part "HTTPS://FAST-QR.COM/" in AlphaNumeric and the rest "blog/mode_indicator" in Byte mode. It saves space in this particular case.
It's always on 4 bits