I noticed in most cases from medium to big size there are usually double zippers. These don't normally appear in small cases. What is the benefit of double zippers in contrast to single ones?
Only purpose I can think of is having the ability to open a certain part of the case (the one in the picture is a laptop case). If so why should this be related to the size rather than the use of the case?
Ease of use & speed
You can easily unzip a small case with one hand, but when the size grows - unzipping it with one hand will not be so easy as you have to turn your hand all around the big case, but the same becomes easy when you use your both hands.
Generally, you put your laptop case on your lap or on table in front of you and use your both hands to unzip it. It would be tough with one zipper and will take more time.
It seems to me the two zipper system has the following advantages:
1) You can localize your opening point. In your example if your want to access something in a top corner one zipper might take two or three corners of travel to make (since it only closes at the ends). Two zippers might be immediate depending on the location you chose. Travel time is typically less with two zippers (and this does little difference for small containers).
2) Two zipper system tends to adapt better for both left handed and right ended persons and the position of the person relative to the container. It is more flexible altogether.
3) In some cases distributing by two zippers the tension applied might be very advantageous (usually school bags or other such where it is not unlikely to be filled with more stuff than its supposed to).
