Salting eggs in brine is a way of preserving eggs, Thailand is very hot and eggs could only be kept for a short time unless salted. As with many cultures, the Thai's acquired a taste for the salted product, and created recipes around it. It's common in Thailand to east rice soup with salty egg, the egg adds the salty taste to the dish! The salt concentrates in the yolk of the egg, the longer they are kept in the brine, the saltier they get, 14-20 days is best, if you leave the eggs too long in the brine, they will be too salty to eat. If you're unsure how salty they are, remove one, boil it and taste it.