What is a unit?
A unit is any standard used for comparison in measurements. Unit conversions allow for measurements of a property that have been recorded using different units (e.g., centimeters to inches).
Why units are important?
When describing a chemical reaction, units provide a standard measure to which we can refer. In other words, we ensure proper comparison between newly collected data and the old data. For example, when we measure the amount of liquid in a container we use its volume. The volume may be in meter cubed, centimeter cubed, millimeter cubed, etc., and all of which may be inter-converted between one and the other. Once we know the volume of this liquid and if someone should ask us how much liquid is in that container we can say there is 100 m3 and it is clearly understood how much liquid we are talking about because we used units. If on the other hand we had said, there is a 100 in that container, that alone would not provide sufficient information on the amount of liquid in the container.