The requirement that we specify correct units with each measurement becomes obvious when we are dealing with quantities that are not part of everyday human experience. The following examples should make this clear ...
We all know that the units for measuring the price of petrol are "cents per litre", even though they are not specified on the sign.
Most people cannot draw on their everyday experience to say whether the units here should be "kilometres", "miles" or "metres". In fact, the sun is 144 million kilometres away from Earth, on average.
We will be covering units a bit more thoroughly later, in the section on S.I. units. You can skip ahead to look at them now, or continue sequentially.