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Standard units like inches and centimeters allow consistent, communicable measurement. An inch is approximately the width of a thumb; a centimeter is smaller. Using rulers develops precision and understanding that different units yield different numerical results for the same length.
You already know how to measure things using non-standard units — like saying a crayon is "5 paper clips long." That works fine if you and your friend both have the same paper clips. But what if your paper clips are longer than mine? We would get different numbers for the same crayon, and neither of us would know who was right. This is the problem that standard units solve. An inch is always exactly the same length, no matter where you are or who is measuring. A centimeter is also always the same. When we both use inches, our measurements can be compared and trusted.
An inch is roughly the width of your thumb — a useful body reference. A centimeter is smaller, about the width of your pinky fingernail. The same object will have a larger number when measured in centimeters than in inches, because centimeters are smaller units. For example, a standard crayon is about 7 inches long — but it is about 19 centimeters long. The crayon has not changed; we just counted more of the smaller unit. This is a key insight: smaller units produce bigger numbers for the same length.
Using a ruler takes practice. Place the zero mark (the very end of the ruler) exactly at one end of the object — not the edge of the plastic, but the "0" line. Read the number at the other end of the object. If the object falls between two marks, read the closest one or the mark just below it. The biggest mistake beginners make is not lining up the zero correctly.
Here is why this matters beyond math class: carpenters, doctors, scientists, and cooks all need measurements that other people can understand and reproduce. "A handful of flour" is not a recipe. "1 cup" is. Standard units are the agreement that makes measurement a shared language rather than a private guess.