What does `-` mean in command `ls -la | vim -`?

Question

I got to know commands like ls -la | vim - can be used to open Linux command output in vim.

1. What does - mean after vim?
2. Is it some kind of special switch?
3. Why doesn't simply using ls -la | vim works ?

Answer 1

from help -- :

Start editing a new buffer, which is filled with text that is read from stdin.

ls | vim doesn't work, because vim reads it's commands from the terminal (as opposed to the content of a buffer)