1. A conjunctive phrase used to emphasize or reinforce a reason, indicating that something is even more true or valid because of an additional factor
She was upset, but more so because he didn't apologize.
Ela estava chateada, mas ainda mais porque ele não se desculpou.
2. Used to introduce a stronger justification or explanation for a statement already made
We should leave now, all the more so because the weather is getting worse.
Devemos ir embora agora, mais ainda porque o tempo está piorando.