If you wait until the very end to add the salt, then the dish is only salted on the surface. The salt will dissolve into the sauce and season the surface of the individual components of a dish, but it won't really permeate throughout.
However, if you salt as you go, then each layer of a dish gets seasoned individually. This results in a more even distribution of the salt and roundness of flavor in the finished dish.
It might sound counter-intuitive, but you'll run less risk of over-salting your dish by doing it as you go. Since the salt is being added incrementally instead of all at once, it's much easier to stop when you think you've hit the right level of saltiness.
Try this: if the recipe you're following gives an exact amount of of salt, divide that amount into portions and add one portion every time you add a new ingredient. If your recipe doesn't give an exact amount, add 1/4 - 1/2 teaspoon of salt every time you add a new ingredient.
For instance, if you're doing a stir fry, add a bit of salt when you start sauteing the onions. Add another pinch when you add the mushrooms, and again when you add the green beans, and so on. If you're making soup, add some salt with each of the veggies, more when you add the broth, and then again as needed while the soup simmers.
And as always, don't forget to taste along the way!
Related: Food Science: On Salting to Taste