We identify things (and understand new ones) by similarities – experience.
Understanding the physical properties of substances helps us to identify them when we find them, and to classify or describe new ones with similarities to ones we know.
For example, knowing that water boils at 100’C and ethanol at 78'C can help us to identify a clear liquid in a container that might be either one. Similarly, knowing the density of different metals can help you to know if you are holding a piece of steel, aluminum, tin or silver.
The Periodic Table of the Elements is probably the best example of organizing substances (elements, in this case) by their properties. It is the “map” that chemists use to learn about new compounds from the known properties of the elements.