There's almost absolutely no reason for a character to do that aside from the author just lazily wanting to change the course of a fight or wanting to tell us the capaicty of a characters in just a blatant tone. I actually can think of two popular cases and one of those you said it, which: Bleach and Jojo.
I watched the last arc of Bleach this year, and there were at least two fights where a character lost simply because they explained their power to the antagonist. In that fight you mentioned, the captain Rose literally told his opponent: "I'll tell you what, my Zanpakuto controls "music" its deceptive melodies resound in your ears... And give me power over your heart" and what did his opponent do? Well, he broke his own eardrums, so the power didn't affect him. There was absolutely no reason for him to explain his power.
It felt like such a poorly executed way to make a character win a fight that it was honestly hilarious. If you don't have a way for a character to understand the enemy ability and make an interesting and in depth combat, just make them spell it out in the most obvious way possible lol, so that you can change the result of the fight to your favor in a one-sided note without anything interesting not tension happening between the two who are engaging in battle.
This will be a bad and even shameless example of this happening, because when a fight happens to have a breaking point just because of this happening is lazy writing. Not only that, but the character explaining his power didn't fit in the narrative and the dialogue of the participants, and it was so explicit about what it tried to do that it felt like just a comment from the author instead of the character.
For me the difference between Jojo and Bleach falls just in how you reach the outcome of a combat and the dialogues; Jojo's dialogue is so over the top and exaggerated that even when the antagonist explain their powers, they do it in a more natural and fitting way. For example, in part 5 when Diavolo explains King Crimson he says: "the clouds of the sky do not realize they have been scattered by the winds! An extinguished flame is not aware of the moment it has been snuffed! It's only the results. Only the results that remain in this world!" which, if you ask me, makes a huge difference as to blatantly saying that his power is to erase time. Not just that, but also it makes more sense as the fight was so one-sided that it was a clear win for Diavolo, so explaining his power, he, someone who is arrogant to his core, makes more sense as to state to his enemy that he is indeed untouchable.
Even leaving that example aside, I think Jojo just does it better because even when an antagonist explains his power, there'e almost another character that does something so random and so bizarre that the fight will not finish just by explaining the power, unlike in Bleach case.
So at the end and for me, it depends of how the dialogue is structured and the tone of the fight, that, and not making the whole "I am explaining my power" the whole reason the fight is decided |