The main reason that it's the best part is how those interactions/relationships can change over the course of the show. Most of the character's relationships have a dynamic range and evolve depending on their actions, just like in real life.
Adora, Bow, and Glimmer begin at odds, but quickly develop a strong friendship. Through Glimmer you learn of her rocky relationship with her mother, but she and Angela grow close. After Angela sacrifices herself to get Adora home
Glimmer blames Adora for not being able to save her; and Adora and Bow question Glimmer's judgment about the Heart of Atheria.
During S4 the Best Friend Squad shows its cracks,
and S5 is when they finally get around to going through the healing process.
The friendship has dynamics and range to it, Glimmer doesn't even know about Bow's dads.
until the show shows you.
You could pick damn near any main character, and their interactions and relationships with all the other characters are compelling.