Because of the ADA, many bigger companies have minimum physical requirements listed for every job description.
The physical determines if a person is capable of meeting those requirements.
if a person has a limitation or handicap that prevents them from donig that AND it is covered by the ADA, the company has to make "reasonable accomodations".
if the limitation is not covered by the ADA the company can use it as a determining factor.
if a person has an existing condition it can end up costing the company a whole bunch of money in lost production, sick time, health insurance costs, safety record, etc.

it's smart business.