Equitable use typifies what we term universal design - being something that can be used by any person of any age or physical ability. A big part of being able to be used is having various items in the home located where they can be reached and accessed by various members in the home, including the very young and the very old and those standing as well as seated.
The whole concept of equitable use is that a person does not need to be of any certain height to function well in their home. It's not a matter of being tall enough to reach something or having enough physical strength or range of motion to be able to open a door, drawer, or window, or or turn on a switch, faucet, or other functional item. Equitable use means that the playing field is level.
One of the easiest ways to illustrate the concept of equitable use is to think of a first grader, say someone 6 or 7, and a grandparent or great-grandparent, say someone in their 90s. Then think of each one using a digital thermostat (as opposed to a round, dial-type, mercury-switched thermostat.
Even with weaker eyesight, the older person can see and read the numbers displayed as well as the up and down arrows for adjusting the temperature. The child - as long as long as they can recognize numbers when they see them - can also use it. They may not know that a 7 and 2 together means 72 and what that means in terms of relative comfort, but they can read the display and also see the adjustment arrows.
The child is relatively short but can still reach and see the thermostat provided it is located no higher than 48" from the floor. The senior person - even if they are in a wheelchair or supporting themself with a walker - can see and read it from that height as well. A child's little fingers and a grandparent's unsteady hand or one stiffened with arthritis can use this device equally well.
Equitable use is found in many other areas of the home, but the digital thermostat is a classic illustration of how this concept exists in universal designed homes. Rocker light switches and other controls (especially dimmer switches and push buttons), lever door handles, single lever faucets, cabinet doors and drawers that open with little physical effort, and mirrors that are hung low enough to be used by everyone in the home (or full-length mirrors in at least one location in the home as a bypass closet door or as a piece of furniture) are additional examples of the concept of equitable use.
If something in the home is hung or installed so high as to restrict its use to just those tall enough to reach or access it, if it takes so much strength or physical effort to use something that only someone strong enough or physically coordinated enough can use it, or it requires a certain level of knowledge or understanding to be able to use it, then it simply is not of equitable use.
Topping the list of Seven Principles makes a lot of sense because of the importance of equitable use for universal design and having items in the living space being able to be used by literally anyone in the home - even if they are just visiting.