Buying a membership gives you access to a pre-set number of additional listings. Additional listings purchased using a membership do not have to be used at the time of purchase, but will have to be used during a specific period of time which varies depending on the membership purchased.
In addition, a membership will give you access to a lot more information about the people who viewed your listings or expressed interest in connecting with you. This will prove invaluable to you as you try to understand user needs and to sell better.