in terms of recipe formulation, you pick your OG based off of style/experience and then brew to that selection.
you check your actual OG, like you said, after all fermentables have been added along with top off water prior to pitching. this gives you an idea of how you did putting the ingredients together (mashing in the case of all-grain brewers), a starting point for monitoring the work your yeast will do, and a way to estimate your abv.
PRIMARY: soon...Dusseldorf Altbier, Ordinary Bitter
SECONDAY: empty : (
KEGGED: 1. SMaSH ApA 2. Cream Ale 3. American Wheat/Rye