I have a shorter explanation if it’s her we’re talking about: she likes to do drawings, lots of people like her posts with drawings, and why not make some money off it. She’s already most of the way to a product and audience. Others trying this might similarly want to test and iterate some free ones on their web site first like she did.
I don’t think it’s useful to guess at her reasons when you could just read her explanation of her reasons.
As an example of why it’s not useful, one of the reasons she mentions near the end is being able to pay artists.