Definition of BIM:

Building information modelling (BIM) is a digital representation of physical and functional characteristics of a facility. A BIM is a shared knowledge resource for information about a facility forming a reliable basis for decisions during its life-cycle; defined as existing from earliest conception to demolition.

BIM benefits:

1. Faster drafting without loss of cost and quality
2. High level of customization and flexibility
3. Optimization of schedule and cost
4. Seamless coordination and collaboration
5. Conflict detection and risk mitigation
6. Easy maintenance of building life cycle

Here is the nice info-graphic which compares BIM based process to the 2D drawings based process:

Learn more about BIM solutions for structural engineers

Agreed with the difference between 2D/3D DWG and BIM. This is in a high need of time that we should adopt BIM over 2D DWG worldwide, to reduce complexity of project,  time projection, cost projection, and overall efficiency of project. In future of touch screens and holographic displays, i hope this will be the dominating option.