The first of these numbers tells the filament voltage. Different tubes were and are used in different brands of amplifiers for their sound charictaristics. The sounds due to the charictaristics of the tubes are substancially different. All of this is due to the type of tube used. This is because transistor technology has taken over for tubes.
Transistors also amplify the signal, but they use current insead of voltage to do it.
This means that a heater is not needed to make them work at maximum output. This is the same reason for the electric guitarist to use tubes.