There are many reasons why a fabricator, contractor, or even engineer would want to use a contract detailing firm. The most common is to supplement existing in-house staff.  A contract detailing company can take care of the little jobs so your in-house staff can focus on that big project, or possibly, the other way around.  Some fabricators choose to use all contract detailing and avoid the personnel issues and overhead of in house staff all together.  Contractors can take advantage of contract detailing on design build projects and cut weeks out of the project schedule by having fabrication drawings ready the moment the steel contract is awarded.  Many engineering firms are developing partnerships with contract detailers to provide detailed drawings as part of the structural design. This may be the wave of the future.  Since erection drawings are in many ways detailed steel framing plans, these drawings can be used as the design drawings. Also, with the growing compatibility between some engineering programs and detailing programs, designs can be directly imported into detailing  software saving time and money.