Think about this: If a designer only gives you the number of stitches to cast on (as opposed to a multiple of stitches), then you're stuck either only using that stitch pattern for that object or you have to figure out what the multiple of stitches are for use on another project.
Designers give multiples of stitches for patterns so you can use the stitch pattern for anything you want (provided you're going from a worked flat pattern to a worked flat pattern or from an in the round pattern to another in the round pattern). As long as your cast on number of stitches equals the multiples + whatever number of stitches, you're good to go.
Personally, I love it when they include this information because I'm often looking for other stitch patterns to use rather than what's provided.
For example, right now I'm knitting a short-sleeved shirt, and I want to add a lace border to the bottom of it (something that's not part of the original pattern). So I look for stitch patterns that give multiples, which makes it easy for me to figure out whether the pattern repeats will fit into the stitches I already have on my needle.
Besides, it's not that hard to multiply (and add) it out to find your CO stitch number!