Any and all kinds of learning are always useful. What is important is to check its relevance, goal and usefulness in the current context of the situation you are in.
Please think through or write down as to what is it that you are trying to learn, what is the purpose behind learning that i.e., is it for your current job, future skill upgrade, learning something completely new, to improve current career opportunities or explore new opportunities etc. Once the purpose or reason is established, the rest of the decisions becomes easy such as budget, mode of learning etc.
You also need to look at the level or grade of knowledge/learning that you are aspiring for i.e., is it beginner, intermediate or expert level, generic or specialised.
On the usefulness of online courses, it is meant for people who do not have the time or because of the geographic spread people cannot access quality courses. It is useful for a generic understanding or a deep dive into some specific topic, for example, you can do a basic course on AI or do a course to learn programming languages in an AI environment.