It's so they fully understand that they're just that "strong"
If u read between the MCs lines towards the old man about "You're only strong cuz yur aiming for the weak." He's saying that not only to the old man but to his teammates too cuz they're in the same mindset as the old man.
They never actually fought someone strong and consider themselves as the strongest.
It's like when you starved a dude by not giving him breakfast and lunch and only dinner. He'll think along the lines of "that was bad. Food is important so I'll manage it for a while'
Compared to a man starved for a week or two, they'll start thinking with "that was awful. Food is important! Let's grow and cook our own food so we'll never starve!" After u fed him.

Had to stop at chapter 17 to comment, too funny to pass by.
Dude really watched everyone getting beaten up and suffering before appearing at the near end and instead of finishing in one blow just spent his time aura farming. Omfg I can’t
and then he got the audacity to ask after killing the evil if everyone is good. Bruh you saw them get their asses kicked one by one what do you mean « Is everyone okay? »