because it makes quantities with a lot of digits small enough that you can do math with them.
example:
say you measure a wavelength in nanometers which is common for wavelengths but then have to use this quantity in an equation with the speed of light; 300,000,000 m/s ..
You then must convert one of these quantities to get both in the same units of measure. Usually you would write 600 nm as 6x10^11 m instead of a 12 digit decimal and speed of light as 3x10^8 m/s
