TL;DR-I use a brewing app to create a recipe that will utilize ingredients I have on hand. I like to brew on the spur of the moment and don’t have a LHBS nearby.
Unless one is set on creating a brand new beer style there really is no such thing as an original recipe; every “from scratch” effort is just one more reinvention of a particular wheel.
That said, if I want to brew a style which I don’t have a recipe saved for in my brewing app (I use Beersmith), I use the “add recipe” function and start mixing and matching ingredients appropriate to that style until the “sliders” on the bar graphs for gravity, bitterness, and color are in the middle of the range of those characteristics for that particular style.
When I first saw this thread I started thinking about how I use the software to put a recipe together and about what I wanted to brew next. It occurred to me that I couldn’t remember the last time I brewed a Brown Ale. I didn’t have a recipe, so I put one together in Beersmith. I brewed that recipe yesterday. I hit my desired volume into the fermenter exactly, and was 6 points higher than the calculated OG, so my efficiency was a little better than predicted. The OG sample tasted great and the beer was bubbling away this morning.
I could have just chosen one of the hundreds of Brown Ale recipes in the Beersmith database, but what would have been the fun in that? Besides, coming up with my own was based on ingredients I had on hand. If I didn’t have the exact ingredients for an existing recipe I would have had to go through the new recipe process anyway, in order to tweak the existing one to utilize what was in my stash.