Associated Business Entities - what is the purpose and the consequence of not entering the data?

 I usuially enter the Associated Business Entities when setting up a legal entity as in the past I have found that without doing so, no tax was calcualted. I have now started work on a new project where the Associated Business Entities were not set up for the legal establishment but the taxes were working fine.
What are the Associated Business Entities for and how do they effect tax calculations and what is the consequences to tax if I do not set these values up?

thanks

Andrew Bohnet