The essence of the paleo diet is to eat foods that the human diet is evolved to thrive on. Humans have only lived with agriculture for around 10,000 years and are not well-adapted to eating grains; hence the focus on animal foods in the paleo diet. Fruit, nuts, and berries also play a part, but are generally seasonal -- we're adapted to eat them, but generally don't get them year-round.
Salt is something else we're not adapted to consuming a lot of. I bet you've seen plentiful mentions of how high-salt the Standard American Diet is; whether it's potato chips, microwave burritos, breakfast cereals, processed meats, french fries, whatever -- it's got a lot of salt. And yet our bodies need salt. What's going on?
Salt and Potassium
Salt and Potassium work together in the body to assist in moving nutrients into and waste out of cells, as part of the sodium-potassium pump. We need both in our diets, as some of these electrolytes are routinely lost in the urine.
High sodium levels cause thirst; drinking water will dilute the blood and restore normal sodium levels. Very high levels can cause confusion and then lead to paralysis, seizures, or a coma. You're unlikely to every experience very high sodium levels unless you're gorging on salt, so I'll move on.
Low sodium levels (hyponatremia) is a more common ailment. Drinking too much water, without replacing lost sodium, thins out sodium levels in the blood. There've been a few highly-publicized cases of teens or young adults dying from taking recreational drugs like ecstasy -- these are actually cases of hyponatremia. They feel very thirsty, essentially overdose on water (dihydrogen monoxide is a killer!) and drop their sodium levels, which leads to confusion, drowsiness, muscle weakness, and seizures. Sports drinks contain sodium to replace the salt that you lose through sweating to help prevent this. Water is great, but if your exercise leads you to sweat alot, you need to maintain your sodium levels, too.
High and low potassium levels are much less common. Unless you're on a diuretic or other drug that dramatically alters potassium levels, chances are you won't run into very high or low potassium levels.
A Taste for Salt
Most paleo foods are high in potassium and low in sodium. Beef, chicken, and pork all contain solid levels of potassium, and many fruits, such as bananas and especially avocados, have very high levels. Paleo man got all the potassium he needed from his diet, but didn't get much salt.
Salt was rare in the ancient world, and I'd guess in the paleo world, too. Our word "salary" reflects this history -- it comes from the Roman practice of paying soldiers their wage as salt. It's easy to see that humans might have evolved a "salt tooth" to encourage them to seek out salty foods since they'd otherwise get so little of it. (Similar speculation suggests that we have sweet tooths for the same reason -- that high-energy fruits were rare to paleo humans, and if we run across some, we should grab as much of it as possible.)
Salt and the Paleo Diet
The Standard American Diet, in addition to high levels of sodium, comes with heart disease, diabetes, cancer, and stroke. High levels of sodium contribute to high blood pressure and stress the kidneys. Is it causal for the rest of that stuff? I don't know.
But I want healthy cells. I want my cells to perform as best they can in exercise, during training, and during competition. And that means watching my sodium intake.