There are a good number of people who, upon losing an argument conclusively, or having been shown to be factually wrong, will respond, "Well that's your opinion." They say this to reduce a serious policy discussion or debate to a matter of personal preference. This is extremely harmful to public policy discussions on any issue as it removes the line between personal preference on policy and serious facts that should guide any discussion on the issues.
To make the difference clear; a fact is either true or false - it can be proven or disproven. "It's raining today" is a statement of fact. Someone can go outside and check to see if it's raining, independent verification is possible. It could be wrong. That's what makes it a fact.
An opinion, by contrast, is an expression of personal preference or assessment. "This weather is awful" is an opinion. It is not possible to prove "awful", there's no concrete definition of "awful". "Awful" is simply a expression of personal opinion. The facts that are "awful" for one person may be "great" for another.
This seems pretty straight-forward but many people will retreat to smearing the difference between the two to conceal the fact that they are wrong. This is often for conspiracy theorists especially, as they'll state their conclusion as hard facts, and handwave contradictory facts as "opinion". Sadly, such foolishness is not limited to conspiracies. Other, otherwise sensible people engage in this as well.
The problem here is that it reduces the ability of people to evaluate facts and make serious judgments on important questions. Once people get in the habit of dismissing actual facts as opinion and calling their opinion "fact", they lose the ability to rationally evaluate policy questions and just scream at each other. It reduces space for serious discussion and create space for conspiracies and paranoia.
Once people get into the habit of placing emotion and desire above reason, both sides of a discussion move into tribalistic and use their positions to show how strong a member of the tribe they are. Such selfishness creates animosity and belligerence, which leads to bad policy.