You already know why your character did those things. What's to flesh out?
It provides the opportunity to see NPC reactions to your reasoning, and probably allows them to debate with your character. Getting other people's opinions on your decisions lets you see them from a different perspective, forces you to argue the merits of your choices, and in doing so you can make additional roleplaying decisions about how they react to criticism, how they argue their case, etc.
You could also retroactively discover additional reasons for your decision-making that you hadn't considered at the time, if, for example, you made a rash or emotionally-driven decision in the moment rather than a carefully calculated one.