They’re on that table so people who only/ mainly climb trad can get some sort of idea of how the bouldering grades relate I think. That feels pretty outdated though because 99+% of climbers become familiar with bouldering or sport grades first nowadays.
British tech grades are basically useless for grading entire boulder problems because they describe the difficulty of single moves (a route or problem with a single 6b crux move gets 6b, as does one where every move is 6b, this is partly why you’ve got the adjective grade before it for trad routes and why the grades overlap so much in that table) and also because, as you say, the harder the moves get the wider the boundaries between the grades become.
The single-move style does make it easy to give people an idea of what a crux move is like “it’s 7A but really it’s just one British 6b move” or “it gets 6C but really there’s nothing harder than British 5c on it” but again that only really works if you’re describing a boulder problem to your fellow grizzled trad mates.
British tech grades have been used for boulder problems in guides in the past but only in the small period between bouldering first being written up in books and B/ Font/ V grades being adopted.
