Shower scenes are always necessary. Always.
Not even little.
You are right, not a little, they are a lot necessary
Look, when something is made up to be ecchi from the start as in with this manga, then whatever have all the shower scenes and bouncing boobs as your story or theme or whatever requires. Knock yourself out. But when you have a work that, ostensibly, wants itself to be taken seriously, random out of nowhere service scenes are nothing else but a sort of bat signal for otaku to reassure them that "don't worry, this caters to you". It also gives the impression that your work is not good enough to stand on its own without appealing to such crowds.
Now, this does not mean that you can't enjoy such things in whatever story you choose to, again knock yourself out. Personal biases do not change the fact however, that more often than not this stuff is slapped on for no good reason and normalized to a terrific degree.
Having said all that...
I don't know if the shower specifically was necessary but that whole sequence served two purposes, to introduce the knowlegde about Kirara and to show that angel-san feels weird when boobs are near her, that's why she had that reaction when her kohai approached.
It was to show that angel san was suffering from the woes of been flustered and attraced to girls but been too embarassed by it.at least thats what I got from it.
Well, that works as well as any explanation I suppose. And as established this work has no qualms about nudity anyway so it is what it is. I am sure funny shenanigans will follow for us all to enjoy!
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