If you built a calc app you'd likely not use it in practical use. The purpose of it is to give you a goal to works towards. It's easier (imo) to think "ok, I want to make a calc app for my phone.... how do I make an app for my phone?" then you search, find the answer, write it up, get it working, then move to the next thing "I have a blank app on my phone now, how do I add buttons?" and so on... And yes, I built my website without knowing a single thing about how to build a website when I started. I simply looked up a good web framework in python (I like python) and went from there. Each time I had a problem I needed to solve, I searched it up and figured it out.