This is how the W3C defines it for WCAG standards. In part this is because good acessibility helps everyone, whether you realise it or not. For example, if you

This is how the W3C defines it for WCAG standards. In part this is because good acessibility helps everyone, whether you realise it or not. For example, if you are in cold weather you have gloves on which limits your manual dexterity - you can't touch the watch without taking off your gloves, or can't turn the dial - in that situation you have the same dexterity as someone with one hand or limited finger grip. The same goes if you are holding a basket or child. Alternatively, you have good eye sight and good hearing... for now - unfortunately this probably won't always be the case. But as you age (or if you go to a loud concert) this can change rapidly - having alternate ways of using technology will come in handy in unexpected ways. Designing for situational disability, ie. I cant use a device in this way right now, will have dramatic impact on users with permanent disabilities - but also helps people with situation disabilities right now. (Also I'm not sure why this comment is being so viciously downvoted, its a good question.)