-(으)니까 supplies the reason for going, and in these lines the reason is usually being defended against an implied doubt: 학교에서 괜찮다고 했으니까 간 건데 (the school said it was fine, that's why I went), 찬희가 불렀으니까 가서 (Chanhee invited us, so we went). The near neighbour -아서 가다 also gives a reason, but -니까 is the one that works before a suggestion or an intention, which is why 있으니까 가시죠 and 싶으니까 갈게 take this form.