The Patch Tool in Photoshop is one of the most versatile, and widely applicable tools in the Photoshop arsenal. It is so easy to implement in almost any type of photography retouching. Speaking from a personal perspective, it is probably the tool I use most aside from the all-encompassing Brush Tool. The ease and speed at which image subjects and backgrounds can be cleaned up using the Patch Tool is beyond anything I can think of in another application.
In this short Phlearn video, Aaron Nace takes the time to explain most of what there is to using the Patch Tool, where its limitations lay, and how to adjust its settings for the right type of environment and results.
What the Patch Tool essentially does is allow a particular, free form selection of an image, to be taken and dragged into another location in the image. That selection is then sort of blended in with the surroundings, edges softened and all, to give a very close texture match to the surroundings. The applications for this are rather obvious, as it lets you add or remove or duplicate portions of the image with speed, and even do it between photos.
I photograph people and aviation primarily. Within these two fields, the patch tool is irreplaceable. For aviation, I often use it to clean up skies, airfields, and even tone/texture match parts of the aircraft, which for example, may be blown out highlights or oil stains on the body.
For portraits etc, the Patch Tool comes in handy almost everywhere. I use it to clean up uneven or blemished backgrounds, and of course, on skin. In fact, it’s no secret that using frequency separation is a primary method I use to retouch skin, and I actually use the patch tool within the ‘High’ and ‘Low’ layers to help even out tone and texture.