The primary advantage of 2-D design is speed and price.
The primary advantage of 2-D over 3-D modeling in structural design is price. Drawings in 2-D are easier and faster to produce than 3-D designs. For most modern projects, 2-D won’t meet long term needs.
In addition to being faster and less expensive to produce, 2-D modeling can generally be done on lower-end computer systems. This can have another significant impact on the cost of creating models.
Modeling in 3-D allows for greater collaboration as designers of various components of a project, such as HVAC or plumbing for a building project, can work on the same model. In 2-D design, each designer or group must create their own set of plans that makes it more difficult to see how each component fits into the whole of the project.
In many cases 3-D models will be required eventually. For services such as purchasing or prototyping the level and types of data included in a 3-D model are required.
In some cases, where different design ideas are being debated, it may be more cost effective to create multiple sets of 2D designs until the range of possible designs is narrowed and then convert the 2D designs into 3D.