Here, you specify the amount of revenue you can expect from the opportunity if you are successful. Inside the full opportunity form there is a field in which you can specify the percentage likelihood that an opportunity will result in a sale. These values are useful in many statistical reports, although they are not required to create an opportunity.
Increasingly, more reports look to expected revenue and percentage of opportunity completions. Therefore, depending on your reporting requirements you may wish to encourage sales people to set target goals for each opportunity to better track conversion.