Buying more resolution than you need may waste time and storage space. And graphics files consume storage faster than any other kind of file except video. The difference is significant when you consider that a letter-size page has 93 square inches. If you're adding a scanner mostly to gain digital copier capability, you also won't need to invest much. If you plan to use the scanner with a fast laser printer and computer to emulate the function of a high-end digital copier, you'll need higher resolution scans of originals to alter image sizes.
Resolution is only one measure of an image's quality.
Depth, the number of shades or colors for each image element, is also important for some tasks. Increasing the depth of grayscale and color image scans increases the file size and does nothing for OCR work. More powerful PCs have also played a role in cutting scanner prices. Scanner makers have shifted much of the necessary processing from the scanner's interface card to the PC's microprocessor. Early scanners either used expensive proprietary interface cards with their processors and lots of memory, or an expensive high-speed SCSI interface card.