Theory is good for understanding how things work.
I don't think anyone has a moral imperative to read theory, just as learning about any other topic isn't a morally good or bad thing to do.
But the political, social, and economic organisation of our world impacts every aspect of our lives. Most people here understand this, understand that morality cannot really be separated from politics in any meaningful way.
So, very often, people will attempt to figure out how things work. To talk through what they think is going on. And what they're doing, is theory. The texts people currently describe as "theory" are mostly doing that, trying to figure out how stuff works and what is and isn't true, and what we ought to do about it.
More often then not, people who aren't very familiar with theory, will end up voicing existing theoretical ideas. That's not a bad thing! I think it's impressive and it makes me feel very optimistic every time I see it happen.
Marx has this idea, that workers, unlike those who exploit them, have an intuitive understanding of the way things work, because they live that exploitation every day. In a way, when people say "I don't need to read what old white men said, I know how I'm being exploited", they're saying a bit of Marxist theory. I don't think that means you have to read Marx. I don't think that means you shouldn't read Marx. I just think that's neat.
Main reason why Marx is hard to read is because in many of his statements he's plainly wrong, but because he very much wanted to be right, he invented a whole new alternative to logic - the dialectical materialism.
When you read Marx, you know he's wrong, but then you're being told that in order to understand the rest you just must disable all critical thinking (because it's "bourgeois") and accept his method.
When not forced to, it's obviously not an easy thing to do.
