Why do people format their hard drive when installing an OS?

[D3lit3]

Just curious, why do people format their hard drive when they wanna install an OS on that hard drive? What does formatting do if the drive is already empty?

[Dtneter]

it cleans up other data that you cant acces i dunno thats what ive heard xD

[venny]

if its a new drive, formatting allows the drive to be usable. ie, lets the data be written in that particular format.
for a used drive, it simply erases all the data on the drive, thereby giving a brand new freshly formatted drive again.