(for some reason I cannot reply on your next comment, there is no reply button) yes, I meant when you need to draw their attention, for example, you see they dropped something and did not notice, or you know they are coming to you but you don't know their name. Thank you.
(there is still no reply button) there ARE cases :) my job is all about working with people, so sometimes I meet them at strange places and sometimes I have to approach them first. Like...you are standing at the meeting spot and you see someone who comes there and starts looking around, obviously in search of something, so you just call them and ask "Hey, are you here for the research?" And yes, since I work with people and social research, I can give you real life examples for virtually any possible and impossible situation you can ever think of :) mostly because I had it happening at least once haha but yeah, I digress, and I got the answer I was looking for, thanks!
Also if you see an interesting person you want to talk to out on the street or on a train or talking to people at a bar or a club or any number of reason one might need to approach somebody they don't know lol. 'Hei du der' does seem to be useful then thank you very much :D