This expresses the speaker’s intention or conjection. Can be used to express an intended action and then request something from the listener.
Alternatively, the 2nd clause provides suggestion/advice, the 1st the reason for such advice.
커피는 많이 마셨을테니까 물 드릴게요.
I’ve drunk a lot of coffee so give me some water please.
이따가 갈테니까 조금만 기다려 주세요.
We’ll go after awhile so please wait just a little bit.
지하철은 복잡할테니까 택시 타고 가요.
The subway is congested so lets take a taxi.
요금 망고 철이라 망고가 싸고 맛있을 테니까 망고를 사 가요.
Since it’s mango seson now, mangoes should be cheap and tasty, so let’s buy some mangoes.