What framework should you learn?
There are a lot of frameworks to choose from. Determining which one to learn is a difficult decision. I have no interest in debating this. They are all just tools. Choose the one you are comfortable with and move on.
Learning a framework provides tremendous value. It helps you get things done, become more familiar with the language, and gain a better understanding of how that type of coding works. Roy has a great series on building web applications with Angular and the WordPress REST API, which is a useful resource for those looking to learn these skills.