They allow you to plug octal base double-triode valves, like the 6SN7, into sockets designed for B9A base double-triodes, like the ECC82/12AU7. Handy for tube rollers provided a) there's room for the larger octal valve, b) the B9A socket was wired for a 6.3V heater (the B9A valves themselves can work with either 6.3V or 12.6V and the amp designer can choose which by the way he wires the socket) and c) the amp circuit can cope with the electronic differences between the valves.VB