Sometimes you don't have the time or opportunity for time-consuming hand-sewing repairs. In that case, iron-on patches can be just the thing for a quick fix for a hole or a stain.
Even though they are designed to be ironed on as they are, I recommend also sewing them on if you have the chance, as it will be much more durable. But you can always iron them on quickly first if needed, and then sew them on at a later time.
It's easiest on a machine since the patches are stiff, and it can be beneficial to use a darning foot if you have one, and then remove the feed dogs, so you can move the fabric/garment in any direction you want. Feel free to practice on a fabric scrap with a pen drawing of a heart or a star or something. The stitches don't have to be very even; the most important thing is that the patch is securely attached :)
Iron-on patches are a bit stiff, so if it's a very large worn area with a small hole, I would recommend attaching a reinforcing patch to the inside of the garment first, either with machine stitching (a bit of a quilt style with stripes or checks, perhaps with thread that matches the garment?) or by using a fabric patch and Vliesofix, and ironing it on the inside, and then covering the hole or stain with the iron-on patch. Of course, you can also use iron-on patches to decorate clothes that have no damage or wear!
Reinforcing the inside of worn areas is beneficial so you don't get a new hole right at the edge of the iron-on patch, where there will be extra stress with the hard patch against the thin/worn fabric.