It shows them that they are the minority. Period.

It also tells them and others who think like them that this ends here and now (before it can gather more steam and be allowed to fester).

With Bannon, Miller, Gorka and the fake prez giving the alt-right right the thumbs up they feel empowered. This lets them know that they aren't going to get away with it from others in the country who do not support them. If you look back at Nazi Germany at the beginning too many ignored it and thought/hoped they would eventually stop and fade away which of course it didn't. No one wants that scenario to happen again anywhere.

Strength in numbers...except in elections where the popular vote doesn't count anymore. We must resist that by a show in numbers too (RESIST!).

the spread of racism, hate and violence towards the powerless is a good thing. I was at a rally Friday in Durham NC. Hundreds of people showed up because the kkk was supposed to have a rally at the place where the confederate statue was taken down. They didn't show so we chanted, danced and met other caring individuals. We sent a message "not here, not today, not in my community".
About 35% of this country still thinks Trump is okay. How many Germans do you think actually thought AH was okay and voted for him? About the same, 36.8%. So if you want to hide away and eat cake (hint - that's not what she was promoting; that's what she was making fun of.) that's fine; so did Chamberlain. That didn't go so well.