Not all "ism" words fit the category. A neologism, for example, is just a made-up word. No, seriously, that's what neologism means. And not all discourses have an "ism" label. When a discourse gets an "ism" label, and the label becomes popular enough that it doesn't sound stupid anymore, it gets easier to discuss and handle in some ways, and harder in others.
An idea included in "conservativism" might get rejected by liberals because, well, it's conservative, see? That's an easy mental step for the liberal, but it's not necessarily the best way to come at it. Should each idea be judged on it's own merits? Or not?
What just happened there was all about labels. It's easy for the liberal to handle and judge the idea quickly, because it's in a package they oppose. They oppose it because they've accepted a different labelled package, and chosen to identify with it and as it. The liberal in this instance has labelled their own person.
(And yes, this works just as well if you swap the labels.)
It also means that a discourse can be attacked as a total thing - sexism and racism, say. They can be ascribed to people - "You're a racist"!
"Isms" make us faster. They can let us identify where someone is going from only a handful of points. They let us identify our own stances, setting boundaries for "I basically trust information from these sources, but not those". These are features.
"Isms" can also bias us into rejecting good stuff out of hand. They can support us in simplifying others into caricatures. These are bugs.
For the record, I'm a syncretic, engaged in syncretism. I just want to be clear, y'know.