Custom Fields are used for tracking additional information that is important to your company pertaining to a Customer, Site, System, Recurring, Work Order, or Proposal, where a field is not provided in the application.
When setting up and using Custom Fields, they apply to every record for a given entity. For example, if you set up Customer Custom Fields, every Customer record will have the same custom fields available.
The data saved is for reporting purposes.
Prior to being able to enter information into custom fields, these fields must first be configured in setup (Setup > Operations > Custom Fields).