... Not having drip edge won't matter. But on a BAD day, it just may.
The purpose is to direct water off the shingles and into the gutter. You don't want the water to run off the shingle and up underneath the shingle, or down the eaves. (The eaves is the 'face' of roof line that is sort of 'behind' the cutter. If enough water was to get under the shingle, it may get in the house. And if it was to FREEZE there, well, ice may form. And when water freezes and thaws enough, it will do some damage over time.
Drip Edge can be installed after-the-fact. You slip it UNDER the felt paper and nail it with roofing nails. And the front hangs in the gutter. And if the drip edge gets 'hung up' by the hangers that hold up the gutter, usually you take a pair of tin snips and 'cut out' the drip edge at that point so it can lay over the hanger (AKA gutter 'nail'.) There are different types of drip edge. I also have drip edge along the rake side of the house too. (The rake side is the part of the roof that runs from the top of the wall up to the peak.