It's true of a lot of things. If you compare it to woodworking or auto repair, you don't learn much by just doing random cuts or undoing and reattaching a part. Having a purpose helps the understanding and retention of that knowledge. And the opposite is true too - a skill or knowledge can fade if not used regularly. I hate going into old code, or for that matter working on part of a car I haven't messed with for a while. I have to relearn and remember what I knew before. Sometimes it comes back fast, sometimes I have to retravel the road of looking it up.