I just want to know the function of it?

From what I've read, it is just a small tank used to save weight?

 

The reason for the cell instead of a tank is that is has a flexible rubber bladder in it that will not split and dump gas in case of a crash. Many are smaller than the stock tank at least in part due to smaller ones being cheaper than larger ones. They also have a foam insert that absorbs the gas so it will not slosh or pour out if it were split.

 

awesome response. thanks much!