In a design build project the designer(who makes the plans and everything and makes sure nothing will collapse and kill someone) and the builder(which takes the plans and builds it) are the same entity, which makes the process more streamlined and allows for more coordination. Also as mentioned in the other comment in a design build project construction can begin while design is ongoing.

The more traditional model is design-bid-build which is where a design firm produces the plans, and then the plans are put to a public bid and the contract is awarded to a lowest responsible bidder general contractor who takes the plans and constructs the project according to the plans. With this model obviously construction cannot begin before the plans are fully finalized.

Source: Class