One joule is a unit of measurement equivalent to a Newton-metre, which is measured by multiplying one newton by one metre, by the way, one newton is about 9.3kg of force.
We need standard measurements because it helps us to find the measurements of things.
You would need to have volume measurements.
The measurements that you need to find the area of a square is Area = Length x Width.
your dick
Measurements need to be specific so we use significant digits.
You need three liner measurements to locate anything in space, which the measurements share a vector. and from these measurements you can trianguate the location of anything.
You need its mass and volume.
You need only one to completely describe its dimensions, although you can make lots of measurements.
Variable
When you need very tiny measurements.
The distance and time.
You should convert metric measurements to standard US measurements when you need to communicate with someone who is more familiar with the US system, or if the specific context or requirements you are dealing with call for the use of standard US measurements.