V. + (어/아) 오다 => "to do . . . and bring" V. + (어/아) 가다 => "to do . . . and take," "to progress/go . . . (ADV.)" -어/아 오다 and -어/아 가다 are useful helping verbs to indicate the direction of the main verb. There is no one-word verb for "to take" or "to bring" in Korean, but their equivalent expressions are 가져 가다 "to have and go" and 가져 오다 "to have and come." Similarly, Korean has 사 오다 for "to buy and bring," 캐 오다 for "to dig up and bring," and so on. 잘 해 오다 to do well and bring 잘 해 가다 to make well and take 요약해 오다 to summarize and bring 요약해 가다 to summarize and take 가져 오다 to bring, to fetch 가져 가다 to take away 사 오다 to buy and bring 사 가다 to buy and take 물어 오다 to bring by mouth 물어 가다 to carry away by mouth (of dog, cat, and so on) 알아 오다 to bring back findings 알아 가다 to take back findings 집어 오다 to pick up and bring 집어 가다 to pick up by hand and take away 씻어 오다 to wash and bring 씻어 가다 to wash and take away 싸 오다 to bring wrapped 싸 가다 to take wrapped 찾아 오다 to come to visit 찾아 가다 to go to visit 끌어 오다 to bring by dragging 끌어 가다 to drag away For some verbs, -어/아 가다 indicates that the action or event is "progressing (well/slowly/fast)," as in 잘 되어 간다 "it is going well" or "it is coming to (an end)," and as in 일이 끝나 간다 "the job is coming to an end."

Compared with English, it's much easier to make a compound verb in Korean, so a lot of single verb expressions in English are expressed as compound verbs. For example, getting groceries is usually thought of as a two part action of buying it at the store and bringing it home to stock it, so we often say 장을 봐 오다, where 보다 corresponds to buying and -아 오다 to bringing it home. This is especially common with the movement verbs like 가다, 오다, and 다니다. Many common actions are expressed this way: - 좀 더 있다(가) 가세요 = stay longer and then leave. - 한 번에 10만원 어치를 사 가는 손님들 = customers buying a hundred dollars' worth at a time. - 한바탕 운동하고 왔다 = I did some massive workout (and came home). - 저 사람은 온갖 거짓말을 하고 다닌다 = He goes around telling all kinds of lies. (There are different ways to connect the verbs like -아/어, -고, -러, -려고, -다가, etc depending on the meaning.) So in your example, 먹을 거 사 오려고 하는데 is the natural way to say it to someone at home. It implies the speaker is going to get something to share it with the person listening. If you just say 먹을 거 사려고 하는데, it might sound like you're thinking of buying it for your own purpose and not for the people at home.