One doesn't always have the time or opportunity for time-consuming hand-sewn repairs. In that case, iron-on patches can be perfect for a quick fix for a hole or a stain.
Even though they are meant to be ironed on as is, I recommend also sewing them if possible, 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 might be a good idea 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 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 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 strain with the hard material against the thin/worn material.