The Gates in question are the boundary between normal and magical consciousness. Just as opening the Circle limits the ritual in space, so opening the Gates limits the ritual in time. Not everyone opens the Gates as a separate activity; opening a Circle can be considered a de-facto opening of Gates, but there are good reasons for keeping the two activities separate.
Firstly, it is convenient to be able to open a Circle without going into magical consciousness; rules are made to be broken, and there are times when something unpleasant and unwanted intrudes on normal consciousness, and a Circle can be used to keep it out - like pulling blankets over your head at night.
Secondly, opening the Gates as a separate activity means they can be tailored to the specific type of magical consciousness you are trying to enter. Thirdly, just as bank vaults and ICBMs have two keys, so it is prudent to make the entry into magical consciousness something you are not likely to do on a whim, and the more distinct steps there are, the more conscious effort is required. Lastly - and it is an important point - open the circle with a martial attitude, and it is useful to have a breathing space to switch out of that mood and into the mood needed for the invocation. Opening the Gates provides an opportunity to make that switch.