I would answer that by saying that what a lot of these groups want is power, and they think that either they can use this kind of language to galvanize people and consolidate a political party or, once they have power—even shared power, as we've seen in Italy—they will then attempt to take over the institutions of the state to make sure that they stay in power. This is what we saw happen in Hungary famously. It's what has happened in Poland, where they haven't succeeded yet, and you can see other extremists and far-right parties attempting to do the same in Europe.
They're interested in putting themselves and their members in charge. They think that they are the only people who legitimately have the right to rule in their countries. Once they are in charge, they will then seek to bend the rules in order to stay in power, and this of course is exactly why they're dangerous to democracy.