Logic circuits have become so complex that they cannot be memorized by a human being. The thousands of details that make up a board design are best handled by computers. An engineer is needed to decide on a design approach and to direct the activity of the computers. If the new board design is like an existing design, the designer can repeat an earlier approach. If a new set of specifications requires using faster or different integrated circuits, then the engineer must consider how the new performance requirements might change the board layout. This usually means starting over.
There was a time when components could be interconnected in any convenient way and the circuit would function. As the rise times have shortened, the problem of handling logic signals, clocks, and power distribution can no longer be left to chance. Today, every aspect of a design requires careful scrutiny. One logic line that is incorrectly connected to a stub can spoil an entire design. One overshoot can limit the reliability of a component. One circuit that is not correctly decoupled can cause a logic glitch. The problems that must be considered extend into board manufacturing. If the circuit boards warp or radiate, the design must be rejected. If the traces are too narrow or closely spaced, the board may be too expensive or not manufacturable.
It is not practical to let a computer simulate all performance aspects of complex circuit and include such diverse topics as cross coupling, overshoot, ...