Why would you use tubes if you're not making guitar distortion?

Do you really need pentodes at the speeds involved? It looks like the guy is toggling a switch by hand to generate a clock signal. Pentodes are devices for RF amplification.

Those 6AU6 tubes are 7 pin units, but they contain only one pentode element. Two pins for the heater, then five pins to hook up the pentode.

There exist common 8 or 9 pin tubes that contain two triodes, with which you could cut the tube count in half. (You wouldn't save any money because those tubes cost about twice as much. But less board area and fewer tube sockets counts for something.