Call is used to call a process from within another process. This is useful for maximizing code reuse and minimizing clutter in the process canvas.
For example, an EASYCommerce site will probably display the price of an item on the catalog, the item main page, and the shopping cart. However, it would be poor programming to duplicate the pricing logic on each of these pages. So, instead there is one pricing process and that is called with a Call process in each of the pages a price is needed.