Why do different teams of designers and subcontractors link and share their models during the design process?
Linking models allows different teams to be able to see what other teams are working on during the process. This is important so that they can see what direction somebody is going in so that they can follow their lead. Also, if one team notices a discrepancy or inaccuracy (or if they have an idea that won't work well with another team's model), they can communicate with the other team to say that before they've gotten too far along in the model. One disadvantage is sharing too early, because the design models will inevitably change. If one team sees a model at one point, and then bases new design ideas of their own off of the other team's model, then the other team changes the original model, the two designs could possibly no longer be cohesive. It's important to share and link models once the basic ideas have been established, but not too early.
How do you think design coordination was done before we started using digital models?
I would assume that teams would have to meet to share their designs and plan allotted times throughout the design process to work together. If they aren't geographically close, they might have to physically send copies of the designs and collaborate over email, phone, etc. This is not very efficient because it slows down the process quite a bit having to wait for both (or all if multiple teams) teams to have time to work together and mesh out problems and discrepancies between the models.
At what point in the project development process should design coordination start?
I think once the structure is set (walls, doors, floors, etc.) as well as structural additions (shafts, stairs, elevators, etc.) is when the design coordination should start. If it's too early, there's nothing to design structurally. Places where beams and columns may go are still undecided so it's better to wait until the designer knows where they should go so that there isn't a lot of change and adjustments necessary. However, if you wait too late, there may not be adequate room for these structural components, and the designer may end up having to move around architectural components in order to account for the necessary structural ones.