The biggest advantage of PoD is to always follow the market exactly. There's no point in pre-ordering hundreds of T-shirts with your motif on a random basis, but in the end you won't be able to sell them all. And you don't have the problem of not offering enough products for sale. 
 
That does everything PoD beginner-friendly. You don't have to invest a large amount, most platforms don't charge a cent to sign up. This means you don't take any risks and can first find out whether there are anyone interested in your motifs. PoD is also suitable for trying things out and observing the market. You can always react accordingly to trends and adapt to your sales. 
 
PoD is also good for establishing yourself in a specific niche. There are always products that can be sold, even if the idea seems crazy at first. This requires extensive knowledge of the relevant platform and your target group. If you already have an understanding and knowledge of commercialization and marketing, you will have a huge advantage over other sellers.