Here are some reasons from the top down!
Brand new shingles, the roof of a new home should be perfect right? I have found roof deficiencies during the inspection of almost every single new home I've inspected. Sometimes it's just simple fix items like exposed nail heads that didn't get sealant, loose nails laying around, etc. I've also found heavily damaged shingles from foot traffic, damages from other tradesmen like in the image above where masonry work was carelessly performed, even shingles covering the ridge vent opening which prevents all the hot summer heat from escaping the attic!
The walls are pretty important don't ya think? The manufacturer of all products have specifications for installation and the installers need to follow these instructions for the products to function correctly and for the desired life span, not to mention warranty for the new owner's! All too often the installers do not follow these instructions for exterior wall products like; Sheathing, House Wrap, Tape Joints, Flashings, Hardie Siding, Stucco, Windows, Doors, and the list goes on and on..... I will not pretend to know all of the product specifications, but I know a lot off the top of my head. I can recognize when there could be an issue and if I don't know the specs for an unfamiliar product I will look it up before I move along.
I will say that a majority of new construction foundation's have passed the eyeball test if I'm inspecting completed homes. If you are having the home built, you should hire an inspector for Phase Inspections. The first inspection of Phase Inspections would be "Pre-Pour", this is of a foundation just before concrete is poured. Countless items are commonly called out that will never be seen again. Rebar in the wrong locations, Post-tension cables incorrectly installed, plumbing and other penetrations not properly sealed in the vapor barrier.
I certainly could keep going, but for now these are just a few of the reasons you should hire a home inspector for your new construction!