SOW/PCR or Billing Rule

Hi

I'm a new administrator and trying to determine if I need to use the SOW/PCR functionality anymore.  I currently create a sow/pcr (invoice settings) and attach that to the project and associate it to the billing rule.  However, the billing rule has a cap on it (the sow amount) and that is what governs our billing and if hours go into overflow, etc.  I'm thinking I don't need a sow since it doesn't govern billing rules.

Does anyone use both of those features?  (SOW/PCR and billing rule)  Or do you just use a billing rule?

My ultimate goal is to create a report showing me funds remaining that is not based on the manually inputted budget (money) field but rather based on the billing rule or sow (many would need to be added for some projects).