The reason this is such a big deal is that when applying to any company, you'll get placed on a team and it's really hard to get enough information on the culture, mission, management, etc. Once you're inside? Tons of information, and a lot of coworkers you can ask. This really reduces the downside risk of a new job.
I know this because I work at Amazon, and switched teams very early. I'm much happier on my second team, and probably would have quit on the first team.