A "welder" can stick multiple pieces of metal together and it may or may not fall apart for some time.

A boilermaker is someone that is expected to Interpret technical drawings, look for flaws in the design, or mistakes, discuss this with the draftsperson and engineer, correct. Nest the material cut list as lean as possible, to save material cost and wastage. Fabricate to the technical drawings specifications. Brace and weld out to weld procedure or the minimum standard, AS/NZ 1554 for structural steel.

When you contract in a well trained/experienced boilermaker you don't just get someone that can weld metal together, you get generations of knowledge passed down through the trade.