There a several moderately complex and variable issues in this question. It is impossible to adequately adress all the variables, but here are some key points.
First there are almost no after market protectors for the rear of the most vehicles. There are some companion rear bumpers sold for "brush guard" type front bumpers. While heavier than stock bumpers they are primarily geared towards providing solid points of attachment for towing and pulling. These will provide little additional protection in the case of a rear impact, and may actually lead to increases in damage (at high speed) as they tend to be mounted directly to the frame, so any shock absorber benefits of a stock bumper are lost.
Grill guards come in two basic styles, "just for looks" and "intended for impact". As you may expect many of the chrome plated grill guards you see in your daily drive are just for looks. While shining does not always mean just for looks, the majority of "intended for impact" bumpers (in my experience) are painted flat black. This is because, you expect them to come into contact with; brush, rocks, trees, earth. Touch up with a can of flat black spray paint is really easy.
The primary identifier of an "intended for impact" front bumper/quard is it's attachment. Let us consider the grill guard pictured in the question. The stock bumper is still in place, so it is unlikely that the "guard" is bolted to the frame, in most of these types of applications it the guard is connected using light screws to equally light body parts. In the event of a frontal impact, the attachments will pull out, tearing the parts of the body, it will then fold up into the grill of the vehicle. All of this happens before the shock absorbers in the stock bumper have the opportunity to do anything. So a collision that may have left no notable blemish on your truck, has torn out part of the lower radiator support, and smashed your grill.
Bumpers/guards designed for impact (some examples) fully replace the stock bumper. There are bolted directly to the frame and are intended to transfer any force applied directly to the frame of the vehicle, protecting the soft parts like, fender, grills and radiators. There are a many options for brush type additions on the basic frame mounted bumper. Personal preference and expected terrain are the primary motivators in the decision. If you are going to be driving up on rocks you might want something like this.
Another body protection option is for the side, often called "Nerf Bars" or "Rock Guards" these are bumpers for the side of your vehicle. These are particularly helpful when you have the possibility of having rocks or trees, impacting the side of your vehicle. As with grill guards these also come in "just for looks" and "intended for impact". If they screwed to tin rather then bolting to a steel frame, you are likely to do more damage by installing something "just for looks". Particularly if it has more impact than someone standing on it to get in the vehicle. A well constructed nerf bar, can actually be used as a fulcrum while in off road situations.
So to answer your question. If you buy and properly install quality protection it can offer a lot of protection. Particularly in low speed high force events. My TJ was 10 years old and not a dent in the body when I sold it.
But if you are in a highway speed accident, little additional protection is offered. These devices work by transferring forces away from light duty body parts to the stronger steel frame.