The glue pen is excellent for attaching patches and appliqués before sewing them on properly. This keeps the work flat, without it becoming too stiff with too much glue of other types, and without things getting caught on pins or safety pins.
It's also useful for sashiko repair 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.
This is a refill for the glue pen; one pack contains 2 sticks.