Free speech, as Mill views it, always helps individuals better understand why they hold views in the first place as well as why others hold seemingly unsavory views as well. If you want to successfully combat those unsavory views in the political sphere, it is necessary to understand the other side; censoring them pre-emptively is ineffective both for your own understanding and in terms of politically opposing them (see also from the other side: bans on teaching anything related to CRT).
Especially in a democratic country like the US that is highly polarized and divided into very different political geographies, supporting free speech is necessary if you want to understand where the other side is coming from better. (See this article about how both sides, but liberals in particular, misunderstand the other: https://journals.plos.org/plosone/article?id=10.1371/journal...)
Bringing it back to race stuff, there's a common story you hear from people on certain parts of the internet: they saw one side that had what looked like a lot of calm, compelling scientific arguments, and they saw another side that never responded with anything more substantial than "you're a bunch of horrible racists whose opinions should be suppressed," and so they decided, welp, looks like these "human biodiversity" people have Important Hidden Truth. And from there some of them can be recruited to white nationalism, which has a similar rhetorical advantage. For anyone who's not a white nationalist, this should be at least a bit worrisome.