This seemingly harmless decision sets off a chain of events that ultimately leads to a major conflict between Greg and his brother Rodrick. Here's why:
* Greg's desire to impress Rowley: Greg wants to impress Rowley, who is trying out for the play as well. This feeds Greg's need for approval and acceptance, especially after his struggles with popularity.
* Rodrick's humiliation: Rodrick, who already finds Greg embarrassing, is mortified when Greg gets the lead role in the play. He fears it will make him look even worse in front of his friends.
* Rodrick's sabotage: Rodrick's humiliation leads him to actively sabotage Greg's play, which creates a major rift between them.
This incident, while seemingly small at first, sets the stage for the main conflict of the story: Greg trying to deal with Rodrick's increasingly antagonistic behavior and the consequences of his desire to fit in.