There are a lot of reasons that people try a paleo/primal diet. But I think one reason trumps them all.
It is (theoretically) the safest possible human diet.
If you have any health condition that might be in part mediated by diet, maybe you should consider trying a paleo diet (including obesity, most people's reason for changing their eating).
There are several nutritional hypotheses for the diseases of civilization and I'll talk more about them in future posts. Almost all of the chronic diseases that we take for granted now (heart disease, cancer, gout, asthma, myopia, Alzheimer's, dental carries, gum disease, seasonal allergies, obesity, diabetes, and on and on and on), either are very rare or do not exist at all in hunter gatherer societies that have been studied. What is more is that this is almost certainly not genetic as the diseases appear very quickly when these peoples adopt a modern lifestyle. People have looked extremely hard for a causative agent, but it is possible to more or less rule out environmental factors like chemical exposures and the like.
So the most likely culprit is somehow the change in diet.
Let's start with the "Paleolithic Diet" (paleo) concept first. Anthropologists know that health significantly suffered with the transition to the neolithic (the introduction of agriculture) and in modern times we have a litany of diseases that we think may be nutritionally mediated. Humans evolved over millions of years to eat a diet much like paleolithic people must have eaten and we must be optimally adapted to such a diet. There has not been enough time on an evolutionary scale to adapt to a neolithic diet completely since we have only been farming for thousands of years. We do not know exactly what paleolithic diets looked like, but we know they must have been extremely varied and we can make a guess that modern hunter gatherer diets are an excellent indicator. All known hunter gatherer diets contain meat in some form, so it must be safe for humans to eat meat. Most of them contain vegetables (green leafy and root), so these in general must be safe as well. In climates where fruit is available it is also consumed, though the quantities can vary from none to small seasonal amounts to year round availability in the tropics. Grains, legumes, and pretty much anything else that needs to be cooked to be safe to eat or edible/palatable (when fresh - not talking about meat here) are not regular staples. Milk products are also non-existent past infancy in the vast majority of cases (there are herders that are exceptions to this).
So the argument for the paleo diet is, you can't go wrong if you eat meat, vegetables, and rational amounts of fruit and nothing else. This diet must be the safest possible human diet and it contains 100% of the macro and micronutrients that humans require. If you believe in moderation in all things, the paleo diet is the very definition of moderate around which you should vary things. Modern diets are very far removed from this and making small alterations to a modern diet in the hopes that you will guess the agents that cause modern disease is a very bad bet indeed - you are statistically destined to fail in the absence of a whole bunch of nutritional data that does not exist.
I have been eating a paleo diet for months now, modified with some dairy because I believe that it is probably safe for me and I have no milk tolerance issues of any kind, but I'm keeping the associated carbs low (I'm eating mostly cultured dairy and high fat dairy). Plus wine, whisky, soy sauce, and occasional very small cheats, including some peanuts and half a hamburger bun here and there. I am not eating potatoes or sweet potatoes or bananas or grapes, all of which are "paleo", but high carb.
I have gone from tipping into the obese category (270 pounds and 28% body fat) to 215 pounds 18% body fat* (so far), so it certainly worked for me. It has also had a ton of ancillary benefits that are obvious. And maybe some that aren't so obvious, but will help me live longer and healthier.
I'm more than halfway convinced that this is the fountain of youth. Come take a drink.
* - Measured by impedance. It might not be super accurate, but you can definitely see that it changed.