The reasons why quite a few students take a gap year are-
Because they have have been in education from 5yrs old to 21 or 22 if they have taken a degree.
They have very little to add to their cv apart from qualifications when they start looking for an entry level position.
A prospective employer would see (hopefully) that they have used the gap year well, in that they may have done some charity work or obtained a job to get some customer service practice, or even travelled to a foreign country. Travel, as they say broadens the mind.
Basically a student needs something to add to their cv to "fill" it out a bit.