I can only think of two, and they would be exploiting this system for their own end. (Also, please keep in mind that at the time of writing I am not 100% sure how kbin & the wider fediverse's blocking system works. If I have some incorrect assumptions please correct me.)
They block the person to get the last word in, thereby "winning" the argument. Silly, yes, but people do this.
They create a post/thread and block anyone who comments/disagrees with them. Those people can no longer interact with the post/thread. The only reason this is a problem is because it is a form of self-moderation. (Though, as @wagesj45 pointed out this could be a self-solving problem if they continue to block everyone they disagree with.)
I consider both of these to be pretty rare exploitations of the blocking system, but they happen. I guess it is a question of is this system worth it with these downsides? Some people will go to extreme lengths to harass people, so I don't feel qualified to say either way.
daredevil