Inches and feet both represent length and can be added, while the unit pounds represents weight and can not be added to length.
Work Step by Step
The reason that feet and inches can be added (after changing units) is because they represent the same physical quantity (in this case length). The reason that feet and pounds can not be added (even after changing units) is because they represent two different physical quantities (in this case length and weight). In other words, if a person is 6 feet tall and 180 pounds, it makes no sense to add "6 feet + 180 pounds= 186 ?" because the resulting measurement has no meaningful physical units.