In short...
The AWS D1.1 code is useful for Customers to specify that the structure they are purchasing is to be built in accordance with.
The reson is that if a structure is built in accordance with these regulations, which reflect the latest in engineering tecnological advances, then the Customer may feel sure that most every precaution has be taken to ensure that the structure will not only be safe and strong, but will also be capable of supporting the loads it is designed to withstand.
Additionally, if the Fabrication Contractor adheres to the Code requirements, and meticulously documents all of the required steps, then he has created lawsuit protection in case some future accident or discrepancy occurs.
Tim
It's all for the customer, just like Tim said. If D1.1 is specified, the customer's building will get Inspected per D1.1, so everyone involved has to meet the specs in the D1.1 code or thier work will be rejected and will have to be corrected. 
Forum
