No one asks a horse to lift its legs higher or longer than a farrier during the trimming process. This can prove to be difficult for geriatric horses that are arthritic or have other medical conditions that limit their range of motion. Farriers can accommodate these horses, while also maintaining our comfort during trimming.
I have found that horses can lift their bad knees (or carpi) higher when I place their opposing foot on a block. Although it does not enable to them to bend the bad knee more, it does allow them to be higher so I can get under them in a more comfortable working position. It results in less stress on the horse and the farrier.