Track every (even the smallest) sale to make sure that you don't forget to invoice your clients.
Sales app lets you manage the sales that you do for your clients. It's a separate part, because you may have an account manager responsible for this and a project manager responsible for the project. This app gives you a quick overview of how many sales have you done, what's their status and amount.
It's a part of the "Complete small business management" pack. If you install the whole pack, you will also be able to link the sales with the clients and see the summary of sales per client as well.