Hello Kad, the answer could be for a couple of reasons.
Most people have a similar overnight pattern, your body shuts down gradually til about 3am, so your glucose levels fall, and then your liver starts dumping glucose into the body ready to fuel the day ahead.
So the best thing to do is an overnight test, which means setting the alarm for 3am, to find out whether your levels are rising higher in the night, in which case you need to increase your basal, or whether you're falling, or even having a near hypo like dip, from which the body is rebounding, in which case lowering your basal might be necessary!
Most people have to juggle their basal to try and accommodate the dips and rises in the night as best they can. (of course, people with pumps can set different basal rates for different parts of the night, but not everyone gets the opportunity of getting a pump, or wants one)