Sewing gauges are very handy. I use them all the time when I'm ironing a fold line or hem. You can run it down the seam as you press and it guarantees that your fold is exactly the size it's supposed to be.
You can also use them at the sewing machine to set up a seam guide. Although you can purchase a sewing gauge without spending too much money, they are very easy to make.
In some cases, you'll want a gauge that is heavy duty and set to one size. The sliding gauges have a tendency to slide (duh) when you're using them, if you're not careful.