As covered the use of a magnum primer is suggested for certain powders, generally ball or spherical type powders. Using the 30-06 as an example since you mention it you may want to use a suggested magnum primer if he powder you are using is Winchester 760, Hodgdon H414, H380 or Accurate 2520 and a few others. The reloading manual will suggest which primer to use and a magnum primer. So in conclusion it is the powder that determines the need for a magnum primer. Loading the same 30-06 with powders like IMR 4895, H 4895, IMR 4064 and others does not require the use of a magnum primer. That is it in a nutshell, there can be more to primers, actually a science unto themselves but just use the suggested primer in the manuals and things will work fine.
Most reloading manuals do cover the thought process behind this in the preface sections before actual load data is shown.
Ron