I agree that the recipe formulation is only one small part of why I like BeerSmith. I like the recipe storage with notes. When I want to make a new batch I can pull up the old recipe, read the notes and very easily generate the new recipe with any changes I want. It also has a huge database of grains, hops and yeast that make life a bit easier. It can keep track of inventory and generate a shopping list for a recipe based on ingredients and inventory. I could do all the calculations of OG, SRM, IBU by myself, but BeerSmith combines them all in one. It took a bit of time to get all my system losses and characteristics dialed in, but now I hit my gravity and volume targets very accurately every time.
+1 to this!
And another +1. I *could* do it all by hand, but having it all in one place works best for my workflow.
The main areas where I use BeerSmith:
1) Recipe design -- having a giant library of ingredients, with automatic calculations of everything I care about, is key.
2) Brew session tracking -- I take all of my notes in the software, and at least for me it's way easier than having it on paper or in separate files. If I save recipes, it's easy enough to retrieve them (and my notes) later.
3) Inventory tracking -- I like to have a stockpile of ingredients, including bulk purchases of base malts, so I find it SO handy to track an accurate accounting of what's on-hand.
I don't strictly *need* the software, but it sure does make my life easier by keeping everything in one place. I will say that I've stuck with the "offline" version, as I have little use (or trust) for keeping my brewing life in the cloud. This is largely the same reason I avoid all of the wifi/bluetooth enabled brewing gear -- I ain't paying good money for hardware that is one Android update or one company bankruptcy away from being an expensive brick.