A standard form of measurement is one where there is widespread agreement as to its value. A non-standard measurement is one which makes sense to only a small number of people. For example, "as big as my garden" will only make sense to people who know me (or can Google my address).
Actually, about 3 countries use non-standard measurements - the SI is the international standard.
We need standard measurements because it helps us to find the measurements of things.
nonstandard and standard measurements...
Five times my [hand] span. Tall as my house. As big as my dog. As fast as I can run. As quiet as a mouse.
SI units