Because that is the time when you will have the most thc content of the day is why some people recommend you take then. If you're clear first thing in the morning then you should be clear for the rest of the day. If you test positive then you're borderline and it would be more probable that you would fail the test in the afternoon.

Having said that its my experience that if I test positive first thing in the morning I usually test negative in the afternoon after a couple days of not smoking. So you will probably be fine just do the dillution sticky too just to be safe.