We do a lot of things during the day and many of those things we do have a specific purpose. Expressing purpose in Korean is not overly complicated, you just need to know the right Korean grammar. One of the forms you can use to express purpose is -(으)러, a form that expresses the purpose of your going hither and yon.
In our daily lives, we go from one place to another all the time. Our going from one place to another always serves a purpose and we often need to inform others of that purpose. In English, there is a way to express that and so does Korean. In Korean, you can use the verb form -(으)러. This verb ending you use specifically to express the goal of you going to a certain place. With it, you inform others what you intend to do there. Because of that specific use, the second clause of a sentence with -(으)러 only uses verbs that express movement like 오다 (to come), 가다 (to go), 다녀오다 (to go and come back), 여행하다 (to travel), 이사하다 (to move (to a new house)), etc. It is why textbooks often introduce the forms as -(으)러 가다 and -(으)러 오다.
가: 과일이 집에 없어요.
나: 그럼, 사과와 배를 사러 근처에 있는 시장에 빨리 갈게요.
A: There is no fruit at home.
B: Well, I’ll go to the nearby market quickly to buy apples and pears.
How To Use
The form -(으)러 is not overly complicated. It follows the ‘batchim/no batchim’ rule in Korean grammar. This means you need to use -러 when the verb stem ends in a vowel (no batchim) and -으러 when it does not. However, because the form starts with a ㄹ, you need to be careful with the ㄹ irregular verbs and ㄷ irregular verbs. Keep them in mind.
Also, keep in mind that in the first clause you cannot use any verb that expresses movement. No verb of movement can use the form -(으)러. But on the other hand, you have to use a verb of movement in the second clause of the sentence. This is important because that is a major difference between this form and the form -(으)려고.