Parametric constraints allow you to force an object to behave the way you want it to. If you need a line to remain vertical at all times, you can set a constraint on it to do just that. Need 2 circles to remain the same diameter? How about a circle for a bolt that must stay 1/2" diameter? No problem with constraints. These are just some examples that will make it easier to control your drawing.
You can draw circles in AutoCAD if you only know the radius or diameter. This tutorial will also show you how to draw circles tangent to other objects.