Is there a reason we use salt and pepper on so many dishes and they can be found in any kitchen table at dinner time? Does it have to do with their flavor synergies, or were they just more available "back in the day"?
Salt is perhaps the most basic and effective flavour enhancer, and so it's fairly obvious why we have it on our dinner tables.
The popularity of pepper is down to the Romans, who were crazy about it. Thanks to the longevity of the Roman Empire, pepper was imported for hundreds of years, helping to establish it as the most popular spice, and keeping the price high.
Bill Bryson reports in At Home that:
When the Goths threatened to sack Rome in 408, the Romans bought them off with three thousand pounds of pepper.
Here is a snarky but historically enlightening article on the combination from Slate magazine.
1) Salt enhances flavors that already exist in the food. Here is an article discussing the science behind the phenomenon from the ScienceFare site.
2) Pepper brightens flavor, and masks off-putting notes, such as staleness or blandness from overcooking. Black pepper was, at the time, cheap to produce, durable in shipping and shelf-stable for a long time, so the Romans settled on that, and we picked up the habit from them. Other cuisines have other condiments for the same effect - in Lebanon, they have sumac upon the table, in Morocco it's cumin, and in Jordan, za'atar. Here is an entertaining discussion on tabletop condiments around the world from Legal Nomads.