As others have said, the genius to Beersmith is being able to plug your numbers in from your system and then being able to build a recipe with your system in place. So your numbers will usually come out close. It's not going to build the recipe for you. I personally find the mac version irreplaceable, but I rarely follow a set recipe.
The other nice thing about it is that I name my recipes with the date I brewed it as well. And save them all. It's a great tool for cataloging all of your previous brews and being able to go back quickly and see what you did.
All that being said, I would definitely look into it at some point, but if you are following previously created recipes, you really don't need it yet. Although, I found it helpful right off the bat. The sooner you are able to start tweeking your equipment profile, the better it will work for you when you start creating your own recipes.
Even though my first 4 all-grain brews were from a recipe, I still plugged them into beer smith and saw how it compared. It really helped me to have my equipment profile set up and ready to go. After each brew I would go back in and plug in all the actual numbers I got during brew day. It does a nice job painting a picture for you about how your recipe will turn out.
So yeah, in conclusion of my long winded post. It's not necessary, but if you plan on moving to building your own recipes one day, you might as well pick it up now. It's nice having a known recipe and then seeing how your brew turns out compared to the kit.