Because we can.
Seriously, the 12BH7 is analogous to a slightly "bigger" 12AU7, and similar enough to be a useful sub. The plates tend to be larger, and while it draws a slightly higher filament current it will not stress the power section. Depending on your system synergy, you might find it gives a ballsier midrange. The tonal character is noticeably different. And best of all, there's truckloads of NOS 12BH7s available for quite cheap. My personal faves are the old New Jersey TungSols. But RCAs, GEs, Raytheons, etc. etc. are out there for a song.