Why is density important for chemistry?
Because we can use this property to determine the amount of substance.
Especially where we have an air sensitive reagent, we can take a measured volume, and thus have a known mass of stuff.
A lot of chemists use syringes and burettes of various volumes, and before using their syringe, they purge the syringe with an inert gas such as argon or dinitrogen. Such an inert gas allows transfer of the reagent to the flask (which is also filled with an inert gas) without its reaction with the oxygen in the atmosphere.