There are several reasons.
Measurement units evolved over time. People used different units across countries over time. Even now, you have the Imperial system that has been mostly abandoned by developed countries across the world. But USA, Burma, Liberia and a few Caribbean islands still use the Imperial system (or a variant).
Differences in scale: It would make little sense to measure the distance between two cities in metres, or the size of an ant. In the first case you would use a kilometre, in the second a millimetre.
Sometimes the way in which a physical measure is derived will affect its units. Energy can be defined as the ability to do work and so the unit is the same as the unit for work: a Joule. However, thermal energy is defined as the amount of heat required to raise the temperature of 1 gram of pure water by 1 kelvin and in 1 calorie.
A unit conversion ratio
It could be the definition of equivalent ratios.
What is a fraction in which the numerator and denominator represent the same amount but are in different units?
No. The same quantities in different units.
In mathematics, two quantities are said to be in units agreement if they have the same units of measure. For example, if two quantities are both measured in meters, then they are in units agreement. If one quantity is measured in meters and the other is measured in centimeters, then they are not in units agreement.
The amount or size of physical quantity depends on two things " number and unit". Every physical quantity has its own standard for fixed amount which is called as unit. The same physical quantity with larger or smaller size is measured in different units. for example 1cm or 1m Therefore even if we have to measure same physical quantity with larger or smaller size of it we should have a standard for comparison.
treu
True. Days can't be converted into gallons, no matter how hard you try.
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity.[1] Any other value of the physical quantity can be expressed as a simple multiple of the unit of measurement.
A unit conversion ratio
Weight is a measure of the gravitational force acting on an object due to its mass. It is typically measured in units such as pounds or kilograms.
There are a few reasons. Historical and cultural: different units were used at in different parts of the world at different times. Many of these "traditional" units were eventually replaced by the SI system. However, even within the SI system, there are different units which are appropriate at different orders of magnitude. For example, the metre is the standard unit for length but it is not the most sensible unit for atomic distances, nor is it appropriate for distances between cities. Furthermore, in astronomy or cosmology there are other units such as an astronomical unit (AU = average distance between the earth and sun), a light year (the distance travelled by light, through vacuum, in one year), a parsec (the distance at which a star appears to be displace by one second of arc between the two extreme positions in the earth's orbit).
It could be the definition of equivalent ratios.
there is no need of medium to travel
work and energy have same unit which is jole denoted by "J"
First the units of the two quantities should have the same category, e.g length units, or mass units, etc. Second: to convert the unit of one quantity to the same unit of the another quantity. third: to perform the comparison.
In the SI standard for physics units (see NIST website,) quantities of electricity are measured in coulombs, same as the quantity of electric charge.