Why do most International System of Measurements contain a prefix?

Answer 1

It is not required to use a prefix. But some of the units are too small or too large for everyday use, so either scientific notation is used, or a prefix for larger multiples (or smaller fractions). An interesting example is the capacitance - the derived unit is the Farad, equal to 1 Coulomb/Volt. But in practice it is such a large unit that the microFarad, or even the nanoFarad or the picoFarad are usually used to indicate capacitance. In other cases, such as length, measurements (or calculations) are done over a large range of values. In the case of length, a meter is a very large unit if you want to measure an atom, and a tiny unit if you want the distance to the nearest star.