Digital products, in my opinion, are the holy grail because they have zero marginal cost of replication. This means that once a digital product is created, it can be purchased an infinite number of times without adding to your workload. Any subject that you're an expert in could be turned into an online course and sold infinitely. (See Jack Butcher - Build Once, Sell Twice).
If you're taking this course, you're getting a first-hand example of me executing this idea. I've been building businesses for over 10 years to the point where I could walk onto a stage with a moment's notice and deliver an hour-long presentation on how to get started with launching a business - I can confidently say I'm an expert in this field.
It wasn't until I took Jack's course that I realised I was also an expert in running skatepark competitions. Having worked in the Freestyle Scootering industry for over 10 years and owning multiple companies in this industry, I had the opportunity to run and host many events at skateparks, literally hundreds. One day, I wrote down everything I knew about running a skatepark competition and pitched it to several youth centres as a course for their students. From a single email, I received approximately $10k worth of interest, which absolutely blew my mind.
The specifics of this example are still pending and processing, but the level of interest alone validated the idea to such an extent that I had to take it more seriously, prompting a revision and restructuring of the curriculum and the offer. More details to follow in a future update.
Anything you know how to do can be converted into a course once and sold infinitely.