Ultimately, the purpose of studying theory is in informing practice. Studying to become a capable revolutionary is like studying to become any professional worker, practice and theory sharpen and inform each other. Theory gets tested in real life by real practice, and is affirmed or denied, then this new theory is advanced and carries onward. The lessons past revolutionaries have taught us from their successes and failures make us more effective.
As an example, in Marxism-Leninism, we find coherent analysis of imperialism. Without properly understanding how imperialism functions, we are blind to why the US Empire is declining while countries like the PRC are rising. When we understand how imperialism functions, we learn how we can kill it. Party structure is another good example, as well as dialectical materialism as a methodology for achieving positive change.
Correct analysis is the basis of correct revolutionary tactics.
As comrade @StillNoLeftLeft@hexbear.net already posted, I made an introductory Marxist-Leninist reading list if you want to dip your toes into theory! The manifesto is a good read, but doesn't really go in-depth anywhere.
