The flatter, properly structured and defect free base will accept wax and glide better than one that is not. A better slide will also help the turn and basically overall enjoyment while out ripping it up on the snow. Keeping the bases in shape also prolong their lives and value.
One common misconception is that a mirror smooth base is the ideal. Try running a wetted finger down a mirror and notice the suction. Compare this to an irregular surface such as some types of shower glass. Notice that your finger moves easier over the surface.
Base structure, like scratches in line with the ski or snowboard, break the suction formed between the base and snow, along with the presence of moisture. Gouges and scratches against the running surface, in varying degrees, create resistance or friction.
A board that is not flat (convex or concave) will affect the ability to get on the proper edge and reduce turning ability.
After getting your bases in shape, after waxing and scraping, it is very important to ‘free’ the structure of the wax left by the scraper by brushing.