>'projecting' everybody onto a single line forces people to 'sit next to' positions they do not appreciate.

That's the point.

If someone has an opinion you don't like but is hard to dismiss or prove "wrong" then you can dismiss it by association with a group of "wrong" opinions. An easy way to make this association is by attributing it to a group whose opinions are known to be "wrong" by whatever the local in-group is. Once you have associated the inconvenient opinion with a bunch of "wrong" opinions it is easy to dismiss.

For example I were to say "every implementation of gun control is inherently racist and classiest" (probably not a welcome opinion around here because most here like gun control) then people would use that to assume that I am anti-abortion, anti-gay, anti-immigrant and all the other things that the HN user-base generally agrees is bad about the modern American republican party and dismiss my opinion.

I could say the same exact sentence in a context where the dominant group is ardently racist and classiest but not pro gun control they would use the bits about race and class to assume that I am some crazy leftist who wants the proletariat to seize the means of production and use the association to dismiss my opinion.

It's quite hard to dismiss a statement like "every implementation of gun control is inherently racist and classiest" when you have to fight it head on. By associating a statement like that with people who also have a bunch of views that are "known to be wrong" by the local majority or in-group the playing field is tilted and it's easier to dismiss.