A fraction is a part of a whole, and fractions are necessary when dealing with portions and amounts that are not whole numbers. Fractions are a part of mathematics and science, as well as simple household functions such as cooking and making home repairs.
Measurement systems exist to easily define portions and amounts of things in daily life. Miles are simple measurements of distances, pounds quantify weights and masses, and cups help to measure portions of food. However, not all portions fall neatly into whole number measurements, and fractions provide the means of measuring these amounts. For example, a precise chemical mixture might require slightly less than an entire gram of a particular substance, perhaps only one-fourth of a gram. A biker might travel more than 3 miles but less than 4 miles, or 3 1/2 miles.
Fractions themselves are specific measurements. Rather than using ambiguous terminology like a pinch or a drop, fractions allow for specific divisions of a whole. Smaller divisions, like eighths or sixteenths, are useful for very tiny amounts, while fourths, thirds and halves help with larger divisions.
Applications such as cooking typically use fractions written out. However, scientific application of fractions more often displays fractions as decimals. Decimals provide a way of displaying fractions as base 10 numbers, like notating 1 1/2 as 1.5.