There’s a philosophical and a practical side to this:

Philosophically, the core of a democratic system is the peaceful transition of power. The idea that you won’t just try to force your will over people with violence and will respect the will of the populace. This is a fine principle in a proper democracy with a fair process and political outcomes that fall within acceptable ranges. If you wanted more money for the trains and someone else wanted more money for the busses, that’s a disagreement you can live with. And if the voting system is set up so you had equal chances both to introduce topics/candidates and vote on them, then great. By accepting the election and not trying to go outside the system to get your way, you keep the peace and allow for that process to be a viable vehicle for change.

If this is a requirement for democracy, then the converse is that if a system isn’t fair and produces unacceptable results (eg, Nazis and genocide), participating in it merely legitimizes it. Obviously nothing physically stops you from organizing, but symbolically you’ve shown that you view the system as the sole legitimate way to exert political power and garner authority. And people will then turn around and say you should vote instead of doing xyz actions. “I don’t agree with your methods.”

On the practical side of this: people put a lot of time, energy, and political capital into supporting candidates in these elections. It eats up the public bandwidth, crowding out other forms of political participation. In addition, once someone works hard to get their candidate elected, there is an impulse, an incentive, to defend them. The people who said to suck it up, vote for Biden, then push him to the left turned around and chastised leftists for protesting over things like the continued anti-immigration policies or the support for Israel’s genocide. US electoral politics is a team sport. People get psychologically invested in their team. They don’t like it when you criticize their team. This makes them resistant to change even on policies they nominally support. I think encouraging people to maintain that emotional investment in elections is harmful. It hinders organizing efforts. It hurts attempts to build class consciousness because it gets people to think about their fellow workers as the enemy and capitalists as potential allies. And the corresponding obsession with 24 hour news cycles turns politics into a TV show. Trying to talk to libs about any history older than like a week ago or maybe at most a presidential term is impossible. If it wasn’t on their favorite TV show it doesn’t exist.

We need to be drawing people’s attention to actual types of political participation. Elections don’t just distract from that, they make people think they’re doing the right thing. It’s a release.

All that said, that’s not to say there’s never value in any part of the electoral system, it’s just very limited. Bernie’s attempts at running were part of what got me more engaged in politics and shifted me from being a progressive-ish lib to being more of a socialist. Important to that though was not just the policy platform, but the structure and messaging of the campaign promoted the importance of mass political participation. I ended up meeting some local socialist groups in the process of going to campaign volunteering. However, most of the time and energy still went into the election only for the system to block us at the end and Bernie to give in. Tons of hours of volunteer time went into doing little more than getting people to sign ballot petitions. We weren’t getting those people into a union or a mutual aid group or anything. We basically just tossed our energy into the void.