>>279877250
>>279877288
Masks are mostly made for us and the plot, not for the characters. Like they wear masks (literally masks) and then they take them off and we see this real face (for example, the most obvious that Uika is Hatsune)

As for why they actually wear them, in the anime world and without deep meaning, well maybe because Saki wanted to hide Mutsumi and Uika face. But i'm also convinced that she wanted to hide own face too
In the original plot Saki seemed to see mygo as enemies, but in the new plot it seems she didn't want her former members to know that she had made a new band