Salt was so valuable for a number of reasons. It was possible to take it from the ocean, but the amount of salt produced just wasn't practical for the time it took from start to end product. Most salt came from mining operations; however, salt was still rare. In the Mali Empire (1230-1600), for example, salt supported a great of the empire's economy, while gold supported the rest. Salt was as valuable as gold in Northern Mali- and was more valuable yet in the south.

In addition, salt was not only used as food; it was also used as a preservative, playing a massive role in storing food safely to keep from starving.

I can't speak to how many, if any, wars were fought over salt but...

a) Although most of the lower classes in ancient rarely ate meat, wouldn't there still be enough naturally occurring sodium in their diet?

It depends on where you live, but yes, in many cases you'd get enough salt and iodine, without adding extra salt to your diet. Switzerland is one example of a region where people sometimes suffered from lack of iodine (which causes swollen thyroid glands, among other things).

Diet isn't the only use for salt though, before refrigeration became available salting was one of the only ways to preserve meat and fish. You can also use it to pickle vegetables (according to wiki pickle comes from the Dutch word pekel (brine), but i don't know how well that etymology holds up. Without food preservation it's a lot harder to survive from one harvest to the next.