I'm not good with analysing literatures, but it brings a parable to
Maihime (which in another novel, a character quickly label the protagonist as "high school girl's number one most hated male protagonist in Japanese literature class", and it explore about how Mizuto became like he is. Also, it gives the push for Yume to finally admit it to herself, she wants Mizuto as lover, not as brother.
Volume 4 shows another painful moment for Yume, when she looked back at the time when she went to another summer festival alone.
Compare to the flashback in volume 1, when Mizuto found her when she was lost in festival (and it's actually the moment he really falls in love to her).