Constraints are nothing new 
One of the latest buzzword is constraints, but it's absolutely not new. Constraints have always been the important part of a layout. You can not make a PCB design that works properly without taking constraints into account.

In the past we simply made notes and had everything memorized during the layout phase.

To day it's quite impossibly to remember and control all the parameters to be taken into account. Fast switching times, high speed signals, impedance control on various layers, combined with clearances, spaces, RF parameters, current, voltage drop, propagation delay, timing, trace lengths, trace widths, impedances, spacing rules between signal types, etc., etc. Now we simply implement all information directly into the layout software, as a way to keep track of the rule sets.

The importance of obeying all the rules is the fundamentals for a good PCB design you can rely on.
GHD wants it Right First Time!

As the final PCB construction is not known before the layout stage, it is not advisable to create a detailed setup of all the constraints in the front-end schematic program. It can cause many unfortunates that either makes it impossible to solve the layout, or leads to an extremely poor result and a subsequent redesign. Further, there is the possibility of over-constraining which is absolute a disadvantage. Many constraints are related to time and hence distance. Only when you know the geometry of the PCB stack up, laminate and component placement, you have the basis for optimizing the constraints. That is why the definition of many of the constraints belongs to the PCB design engineer who sovereign defines and adjusts the detailed constraint setup. 

Constraints is not that easy to setup, but GHD have a whole new way to do this in OrCAD Schematic and on to the Allegro. See related info on right.