Definitions are an integral part of life. They provide and ability to communicate. Without them, the words I just wrote would be meaningless, because none would be defined. And if none were defined, what I wrote would mean something to me and nothing to anyone else, unless, of course, I shared my definitions with the world.
Noah Webster did an amazing thing for the English language–he took the words in existence and gave actual and reasonable definition to them. By doing this, he slowed the evolution of language and created a better environment to share ideas.
The same thing is true of measurements. Architects can design buildings that builders can build (awkward, huh?) because they all share a common system a measurement. If a foot is six inches long to some people and 12 inches long for others, what does it mean to anyone? Nothing.
This all reminds me of the importance of choosing your words wisely. If you define a word differently than everyone (or most everyone) else, then you should be extremely hesitant in using that word. If no one has any idea what that word means, or even worse, if they define it differently, you are putting yourself in a situation where people can misuse or fail to understand your comments. And that’s not a good deal.