In this thread, I'm going to explain as simply as possible why caucuses are preferrable for people on the left:

Imagine a city, population = 100,000.

In a primary, 30,000 will vote.

In a caucus, 3,000 will turn out.

You have two candidates, Jack and Jill. Jack has a ton of money, rich friends, and represents the more conservative interests in the city.

Jill is an organizer, fighting for workers, students, the environment, and queer folks. Most of those people don't have money to give her.

In the primary option, Jack, because he has a lot of money, can reach out to the whole city. He can send mail to every house, maybe run TV ads. Plus, because a majority of people who vote without prompting are more likely to support him, he's at a major advantage.

In that same primary, since Jill doesn't have much money, she can't send mail or run ads. Plus her younger, less wealthy supporters are much harder to turn out than Jack's.

There's a lot of energy for her campaign, but she loses the primary.

Okay, let's look at the caucus option.

3,000 people usually turn out to caucus. Jill's volunteers call all of them, and they learn that 1,000 are going to support her, 1,000 are supporting Jack, and 1,000 are undecided.

Jill's volunteers work on persuading who they can, and they work to turn out more people who will support Jill. They are organizing.

Jack has money, but it's harder to use money to persuade people to come to caucus than it is just to get them to cast a vote.

Jill gets a ton of people to come to caucus night. Because her people were brought in through organizing, they are good at getting elected as delegates to the city convention.

The process is hard and a little arduous, but her people were prepared.

Before city convention, Jill's team, which is even stronger now, is able to have conversations with even more people. Her delegates are bought in, and organized, and because they are good at persuading delegates, they win the convention.

Because Jill won the endorsement, she now has a really strong shot at winning the election. Other endorsed candidates can no longer work with Jack.

Jack might not even run.

The caucus system leveled the playing field a bit, and now Jill has a strong path to win.