PRYM glue pen with refill is great for attaching fabric together before you sew, by hand or by machine. Whether you're attaching patches to the underside or top, a glue pen is practical for holding the pieces in place, without having to worry about sharp needles or getting your thread caught in them. Generally, it's easier to sew through fabric without any glue at all, but a glue pen is a good alternative to, for example, fusible web or spray adhesive, which will adhere to a much larger area of the fabric. With the glue pen, you can be more strategic and economical with the gluing! The glue washes out. It's yellow when you apply it and becomes transparent when it dries.
It's also useful for sashiko repairs when sewing from the inside, because you can glue down raw edges on the hole so you don't have to look at the front while sewing, even with holes and loose threads.