Why do we need S.I. units? Can't I just measure in steps?

Imagine if you went to the market to buy fabric, and the seller measured it using the length of his own arm. Now imagine you send your younger brother to buy more, but his arm is shorter! You would get less cloth. Confusion everywhere!

That is why we need S.I. Units (Système International). It is a global agreement so that "1 meter" in Lagos is exactly the same length as "1 meter" in London or Tokyo. Standardization ensures fairness in trade and accuracy in science. So while you can measure in feet or steps, for science, we stick to the S.I. units.