Beersmith is really great on a lot of different aspects for me.
My equipment - Set up a custom system, and identify the different aspects of your equipment; Mash tun size, weight and material, deadspace, evaporation rate, cooling loss, tub loss etc...Once you know your system and average brewhouse efficiency, it is hard not to hit correct temperatures, S.G. and volume.
The water and temperature adjustments for infusion mashing and sparging corrected for my system make things as easy as measure.pour.stir.
Obviously the recipe formulation as others have said.
Inventory is huge for me. Let's me see what I have of each grain for recipe formulation. It helps when you are running low on something and can adjust a recipe.
Allows me to store the malt analysis for recently bought grains, and AA% for hops.
The brewsheets are also very helpful
Other cool things to check out - hops age, shopping list (if you like to plan brews ahead), recipe conversion...etc...