You don't always have the time or opportunity for time-consuming hand-sewn repairs. In such cases, iron-on patches can be perfect for a quick fix for a hole or a stain.
Although they can be ironed on as they are, I recommend also sewing them if you have the option, as it makes them much more durable. However, 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 iron-on 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 ballpoint pen drawing of a heart or star or something. The stitches don't have to be perfectly 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 reinforcement patch to the inside of the garment first, either with machine stitching (a bit like quilt-style with stripes or squares, perhaps with thread that matches the garment?) or by using a fabric scrap and fusible web, and ironing that 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!
Reinforcement on the inside of worn areas is beneficial so that you don't get a new hole right at the edge of the iron-on patch, where there will be extra stress with the stiff material against the thin/worn fabric.