-러 gives the purpose of the coming or going: 놀러 나오다 "come out to have fun", 보러 나오다 "come out to see something", 산책하러 나오다 "go out for a walk". In entertainment, 나오다 also means appearing on a show, so 홍보하러 나오다 is "come on the programme to promote something" — several of these lines mean that rather than physically stepping outdoors. -러 attaches only to verbs of going and coming; for other purposes you need -려고.