If you do woodworking and DIYing in inches, a solid understanding of fractions is essential. Being able to calculate that half of 4 1/4" is 2 1/8", or that 1 1/2 + 1 3/16 = 2 11/16" is basic shop math that will keep your projects moving quickly.
But often, bringing fractions into the process is, well, completely unnecessary. Let's say you have a board that you'd like to divide into equal parts. You could measure it, then bust out a pencil, paper, and the calculator app, and eventually have to Google a decimal-to-fraction converter to figure out the size of each section. Then, you'd have to find that crazy number on your ruler, and carefully add the units together to mark out your parts. Or... you could just do this.