The point of adding hops at different points of the boil is to get the various qualities of the hops in your beer. Hops contain Alpha acids located in the Lupulin gland. The longer your hops are in the boil the more Alpha acids you will extra from the hops. The more Alpha acids in your brew the higher your IBUs will be.
All your hop additions change from one beer style to the next. Why? I will give some examples.
Brown ale will have most of the hop additions at the 60 minute mark and few at the middle and end of the boil. But why is this, you ask. Brown ales have a profile emphasizing on the malt over the hops. Lower hop additions in the middle and end give a lower to no hop flavor and aroma.
India Pale Ales are big on bitterness, hop flavor and aroma. IPAs may have a 90 minute hop addition to extract as much of the alpha acids as possible. For the flavor there will be large hop additions and for the aroma the additions can be from large to mammoth amounts of hops. IPAs will be dry hopped to bring the aroma to the next level.
Below I have put in a chart so you can know when to add your hops on your next brew day.
