You can associate automated Campaigns with Opportunities to identify which Campaigns are the sources of your Opportunities. In the campaigns; you can calculate the revenue generated by all of the Opportunities associated with the Campaigns.
From an open Campaign; you can view the Opportunities associated with the current Campaign in the Opportunities tab.
You can assign Campaigns to Opportunities manually at any time after creating an Opportunity. You can also assign Campaigns automatically to new Opportunities by specifying settings in your user preferences.
The system administrator can set the Campaign field as a mandatory field to ensure all Opportunities are associated with Campaigns. You may want to have a generic Campaign to associate with Opportunities that do not correspond with existing automated Campaigns. For example; someone can create a campaign named "None"; which doesn’t contain any Campaign activities for this purpose.
You must have Read permission for Campaigns to assign a Campaign to an Opportunity. If the Campaign field is set as a mandatory field; you cannot create or modify new Opportunities without this permission.