Plate carriers come in several flavors.
Large IBAs (Improved Body Armor) - known by many names like IOTV (Improved Outer Tactical Vest) - which have kevlar inserts that cover entire front and back torso (Level II or III protection - handgun protection) and pockets in front and back for curved ceramic or steel plates (level IV protection rifle protection). Then there are plate carriers aka turtle shells which are just designed for carrying the level VI plates.
The carrier runs around $100, plates around $50-$150 each, and you can customize it to your liking.
Quite common, and worn by any professional forces expecting contact with an armed enemy - military, police, etc. They have documented saving thousands and thousands of lives. Why would you NOT want one, if you expect contact with an armed aggressive enemy?
Considering a ceramic plate in a carrier or IBA will cover over half of your immediately vital areas and they are affordable and available, they make perfect sense for anyone interested in home security. Non-vital hits will give the individual time to survive that critical hour until he reaches surgery if he's hit (non head shots of course). Sure, head shots are possible, but let's ignore that for the time being (head shots are beyond the aiming capability of all but the most trained individuals during a firefight, and are luck). This is about minimizing exposed vitals, not eliminating.
A good plate carrier with plates will stop 7.62 rounds, and cover chest, lungs, heart, liver, kidneys, some of the stomach, etc. The kevlar inserts give greater coverage for 9mm handgun rounds). Properly used, you've eliminated all immediately deadly vitals except shots to the head/neck and some large leg arteries.
It's the same mentality as anyone owning a home defense firearm(s). We live in a violent and unpredictable world, and home invasions are a reality of life.
Secondarily, you can store other life saving gear on it, like back up handgun, extra pistol or rifle mags, shotgun shells, zip-cuffs, flashlight, knife, and medical kit if you are wounded in a home defense fire fight (tourniquet, antibiotics, gauze, etc).
With practice, a ceramic plate carrier can be donned in a few seconds for defense of home in a home invasion scenario. Given the option, why would you NOT want to have one on if you hear trouble at the front door (whether you are the 'go inspect' or 'hunker down' mentality?
Would also be great to train your family to don them and take cover in the event of a home invasion emergency. Bullets go through walls ya know.