To sum it up, scouts can be used for:
Find ruins (20 culture, 1 population to your capital, free tech, enough faith for an automatic pantheon, fast archer and so on), it can be very useful at start
Find city states: gold, it's not much but it's never bad to have more at the beginning.
Find natural wonders: happiness, and you can never have enough.
Find all other civs: better to make friends early on far away lands. More civs you know better chances to make profitable deals. It is true however that if you just wait they will end up knocking at your door by themselves, however if you can reach them faster it's better. You can sell open borders to them, and with 10 of them it's a free settler. Of course those 50 gold are mostly profitable on early game, but not so much later, so time is of the essence.
Exploration: The more you wait the hardest and/or more costly will be to know everything about the world. Other civs will expand quickly and will claim lands within their borders. If you want to scan the land for free you need to do it as fast as possible. Remember: knowledge is power.
Barbarian camps: This is pretty relative because you will hardly send your troops on faraway lands, and you can use your starting warrior to scan for nearby camps. It doesn't hurt however to know for nearby camps as fast as possible.
Reasons not to build scouts at the very start:
Archipelago and tiny island maps: An early scout will deplete its usefulness pretty fast. For mapping purposes it's better to build a trireme or two. Even with embarkation your scout will be most likely be killed by barbarian ships. Not worth it. It's better to build a strong unit later on to get to still unexplored ruins while fending off the barbarians that protect them.
Small and crowded maps: Not a standard configuration, but in such cases you have a diminished return on ruins found and you might not need a scout to explore the whole map.
Early conquest plans: It's rarely advantageous but if you happen to be Monty or another civ with strong early units you might decide to skip on exploration to conquer a nearby civ or CS as soon as possible. This might end up being a good trade off. Askia might also benefit more from an early army if a lot of camps are nearby.