Really, the main reason we say, is so we can schedule ahead of time meet ups with other characters from the series.
If you don't want to tell anyone, and you don't like to plan ahead of time to meet people and have fun, that's all you. But don't act like we are all weird for wanting to meet people that like the same things as we do. I'f I say "I'm going as Naruto and want to take pictures with Sasuke's" then I can get people to answer that and say "I'm a Sasuke cosplayer, let's meet up and become friends". But if I don't, then how would I know that person was there? Also, getting things together for photoshoots, it's a good idea to know who is cosplaying who ahead of time so that the people going have plenty of time to think up photo ideas when the time comes instead of things being hectic..We can't say "Let's do a group of Sora and Kairi cosplayers" if no one knows they are there..
The whole "girls meeting at a party wearing the same thing" is completely different (and 99% of the time not actually going to happen) so that argument is literally invalid here..A lot of people say who they are going to be specifically to find others in the same character sometimes.
And as for your friend..Did you ever think maybe she asked because she was wondering if there was a series you were both cosplaying so maybe you guys could match? If you didn't tell her, or were kinda hush hush about it, she might have thought you didn't want her bothering you, then stopped talking to you because it's not like it's a huge deal..There are plenty of reasons to ask you what you are cosplaying.
That couple thing you mentioned under the part about not telling her..That COULD have been her reason for asking you..Before thinking it's weird to ask, maybe think about any other possible reason..
So I can understand surprising people, that's fun yes, but be prepared to not have any plans with anyone because no one knows you are even doing it..Even more so if it's a cosplay that might not be from something popular..There might be all of 5 people from that series..If no one tell anyone they are cosplay from it, those 5 people may never meet the whole time.