In most of courses like medical ,engineering, undergrad and postgraduate , college counselings are engaged so a student can know about the various courses offered by different colleges where he or she can pursue there career confidently. Usually a lot of colleges participate in counseling and a particular college engage counseling on behalf of other colleges.

Let me give an example like for engineering in India, IITs engage counseling every year so that a student can fill his/her college choices. We have around 16 IITs and 9 aiims all over india, so he/she has give a preference of colleges. Like most proffered college first.Then there are around 10-15 courses offered by college in various streams . So student has to give a choice of course also.
 Students and their parents takes a lot of time to to go through different courses and colleges. So a student need to plan well and should take advice from their seniors. 
A counseling can engaged multiple times. They do this whenever their seat remained unfilled and they have a waiting  list of aspiring student who are willing to take admission.

counselling can take place a number of times untill all the seats are filled up , colleges often do have spot counselling too.

Hope it helps.

Thanks.