How could having a knowledge of density be useful in everyday life

Well, let's say you wanted to know if something was made of pure silver or pure gold. Let's use silver in this example.

First we would weigh the silver, let's say it weighs 2.56g.

You could take a container of water, measure the initial volume of water (let's say you have 50mL), and then drop in silver item. You'll notice that the water line rises. Let's say it moves up to 60mL.

Subtract 50 from 60 = 10mL

mass/volume = density

2.56g/10mL = 0.256g/mL

Now you could look up the density of silver. (Ag = 10.49g/mL)

So then you would know, it is not pure silver.