Reason is a way to think properly while avoiding obstacles like bias, faith, emotions and beliefs. Reason is ultimately a tool for finding the truth. Instead of giving one person the authority to declare what is right and what is not, reason divides that authority among individuals.
Neither knowing the truth makes one reasonable, nor not knowing the truth makes one unreasonable. It's the thought that "The person I am engaged in a discourse with might know something I do not. I might be wrong. I might learn something new." As proven by Machine Learning in digital systems where neurons execute over datasets of millions, "On a large enough scale, quantity compensates for quality." Thus, reason helps make societies where laws are fair, equal and enforced on all.
In a reasonable society, we take risks every time we make a statement. If the statement is verified, we gain trust of others or establish ourselves as liars otherwise. But the statements are not always verifiable, in which cases the liars would have a hard time convincing people of their occasional truth than truth tellers of their occasional lies.
The modern age of technology makes connection to the entire world incredibly easy. While it exposes us to people of vastly different cultures and beliefs, it also allows us to connect with millions of other people that think the way we do. People of the latter kind end up in an echo chamber that amplifies their thoughts ten folds. This pushes them further into the cave of ignorance and farther from the truth reason promises.
I have lost track of how many times I've seen a celebrity say things like "[Name] is a horrible person" without stating why they think that, which results in a backlash from supporters of the person referenced, which are again offensive statements without reason. This continues until the original poster claims the backlash is because of their race or gender and does still not explain why they said what they did.
Actions such as these are not only condescending but also very counter-productive. They do not encourage improvement while discouraging current behavior. The next time someone claims on Twitter, that someone else is a terrible person, don't reply to them saying they are a terrible person for posting this. Instead, ask for their reasoning and proof so you can dismiss it or believe it based on your criteria of truth.