> And the judges of these tournaments not only understand it too (I can understand an opponent understanding if they've practiced the same thing)
Generally yes, although a good team will slow down and speak more or less like normal people if they have a so-called "lay judge" who wouldn't be able to understand them going at full speed.
> but seriously value it in scoring?
You don't really get "scored" on how fast you speak (there's no points system), but, as I mentioned, there are strategic reasons to speak quickly.
For instance, a time-honored strategy is to spew out a huge number of roughly-orthogonal arguments (e.g. "my opponent's policy failed to support the resolution we are debating this tournament, and thus shouldn't win for procedural reasons." "my opponent's policy would destabilize the Kashmir conflict and thus lead to global thermonuclear war." "my opponent's policy would preclude this alternative policy I am now presenting, and my policy is better, ergo my opponent's policy is bad in terms of opportunity cost." and so on), and then circle back later in the debate and further develop any arguments your opponent failed to adequately address (perhaps because they can't speak as fast as you).
An interesting counter-example from when I was actively debating is that at least one team on the national circuit was arguing (somewhat successfully, if I remember right) that speaking fast was a reason to actively vote against a team. The rough gist of the argument was something along the lines that being trained to speak quickly (and have the huge amount of prep-work required to really get value out of the skill) was something really only accessible to affluent/"privileged" kids (although that latter term was a couple years away from entering the common lexicon, I think), and then connecting it back to the central topic of that debate season so as to undermine whatever position the other team had originally presented (but, of course, pointing out that the impact of their argument was occurring in the real world, right now, contra the assumed fiction of their opponent's policy proposal or whatever, and thus a more urgent reason to vote for them).
> what is the point of winning a debate tournament like this, or that values this 'debate'?
For the most part, it's a fun and challenging game for the people involved, the same reason people play chess or go bowling. There's a lot of work and creativity that goes into preparing for a tournament, and the debate rounds themselves reward being able to think quickly on your feet and work well with your debate partner. You get a lot of practice at speaking in public, to a hostile audience no less, which is imo an incredibly valuable life skill (and can be very exciting).