When you are arguing for something, you need to "sell" your argument to others. This ability is incredibly important to the effectiveness of your argument. As much as we would all love to be able to just drop facts on people and have them listen, the truth is that the facts often do not speak for themselves. As a result, we must speak for them.
When someone says that you need to "sell" an argument, that is what they mean. They don't mean that you see some benefit.
As far I'm aware I'm free to use my free-speech to post whatever I want. Like everyone else I choose to post what resonates with me and everyone else has the choice to read, watch, listen to what I post or not. But just because YOU disagree with the content I share and my form of activism it doesn't mean that EVERYONE will disagree.
You can post whatever you wish. If you can't get people to even watch what you're showing them, let alone look at it (as you mentioned with "Cowspiracy"), then their own practices in a critical manner, you are pissing into the wind, and not being a good advocate for the very cause you support.
I'm not even talking just about the cause of animal rights, I'm talking about any argument or advocacy you do anywhere. What good is your argument if all it does is preach to the choir?
I'm for animal rights (to a point), and I think a lot more people are than would admit it, because the words "animal rights" bring up images of inadequately informed rich people who live in an echo chamber of their own experience. This image is reinforced by the documentaries that you've posted - They don't talk about the political and societal realities of choosing to, or choosing to NOT eat meat. They don't talk about the perception of meat as a representation of wealth, and what the political repercussions are for removing the availability of cheap, factory meat for people who can't afford to buy properly raised meat. Just saying "eat meat less often" is removing their agency in their own diet, and in their own lives. Yes, a vegetarian diet is often cheaper, but what does it mean, in America (or even in North America, or "first world society"), to be unable to afford to eat meat if you choose? Those are the things that NEED to be covered in documentaries that are sorely lacking.