A pre-drywall inspection occurs at a time when the structure of your home is visible and not covered by drywall and other wall coverings.  Has the plumbing contractor damaged framing during the plumbing installation?  Is a shower enclosure being installed against an exterior wall with no insulation in the wall?  These are just a few of the problems that will be discovered during a pre-drywall inspection that would not be discovered later in the construction process.