It is used to add a more detailed explanation in the latter clause about the act in the preceding clause.
제가 가끔 학교 앞에 있는 식당에서 점심을 먹는데 그 식당 음식이 아주 맛있어요.
I sometimes eat lunch at a restaurant in front of my school and the food at the restaurant is very delicious.
친구가 요즘 기타를 배우는데 아주 열심히 배워요.
My friend is learning to play the guitar and he is learning so hard.
저는 매달 한국 영화를 보는데 한국 영화가 너무 재미있어요.
I watch Korean movies every month and they are so interesting.
강아지가 참 귀엽군요. 언제부터 키웠어요?
The puppy is so cute. From when have you raised it?
몇 년 전에 제가 집에 가는데 저를 따라왔어요.
Several years ago, I was on my way home and it followed me.
Wasn’t there its owner?
네, 주인을 못 찾아서 제가 키웠어요.
No, there wasn’t. I have raised it because we couldn’t find its owner.
따라오다 to follow
주인 an owner
찾다 to find
기타 a guitar
매달 every month