Usually it’s done to increase power output either by running them in parallel to increase current output, or just in multiple gain stages to increase Voltage then running that through a transformer to get the current.
I have a pair of 130W Tube monoblocks each one takes 2 pre tubes and 6 power tubes, and will happily heat a fair sized room in Winter.
You can also use output tube in parallel on an OTL design to better drive speakers directly or low impedance headphones.