으니까 하다

do it because that's so

-으니까 gives the reason as the speaker sees it, and unlike -어서 it can support a command or a suggestion: 지효가 듣고 싶은 말이 있는 거 같으니까 해줘! "she clearly wants to hear it, so say it". 잘됐으니까 하는 얘기지만 is a set hedge — "I can only say this because it worked out in the end". 있었으니까 하지 means "they wouldn't call it that for nothing", i.e. there must have been a reason behind it.

Register: casual spoken