Rachel from House of Pinheiro did a brilliant post showing a plethora of uses for a sewing gauge; there;s bound to be something you've never thought of. If I was a click bait content writer, I might say, "You'll never guess what the hole is for!" and an afternoon that you thought you'd spending sewing has disappeared into an apparently endless search through 40 photos to find out exactly what it's purpose is.
I bought the 'Seams Right' tool by Clover because I had one of these (cheap, cheerful and useful) but also unfortunately made of plastic and last seen melted under my iron with rather devastating results for the soleplate. So this is a sturdier metal version, a bit like a mini version of the Dritz Ez-y hem. It's very handy for checking seam widths, especially if you sew for many purposes- ¼" for patchwork, ⅜" for knits, ⅝" for clothing etc and the buttonhole gauge is useful too.
My first use for it was when binding a quilt- it's just right for measuring the last ¼" along an edge at the corner where you stop and mitre the binding before turing.
I have an appliqué project planned and I though I'd use it a lot for that - I can eye ball a ¼" but I do like to check, either when I'm adding a scant ¼ seam or even if I'm hand quilting and sewing ¼" from a seam line. I've added links for various shops to buy online, including sponsors but they are generally widely available.