Sewline glue pen with refill is great for attaching fabric together before sewing, by hand or by machine. Whether you're attaching patches to the underside or top, a glue pen is practical for holding pieces in place, without having to worry about sharp needles or getting your thread tangled 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 attach a much larger area of the fabric. With the glue pen, you can be more strategic and sparing with the gluing! The glue washes out.
It's also useful for sashiko repairs when sewing from the inside, because you can glue raw edges to the hole so you don't need to see the front while sewing, even with holes and loose threads.
It's also useful for sashiko repairs when sewing from the inside, because you can glue raw edges to the hole so you don't need to see the front while sewing, even with holes and loose threads.