Intelligence on what other firms are doing, even if you are just getting a very high level view. Around 2009, I was working at an algo trading startup, and long story short, after having a previous company call me up after their non-compete in a business ended, and in the process one of them slipping in that they made like 4x what I was making at the time, I decided to look around.

I got lots of bites on my resume! And I was called in for lots of interviews. But they didn't really go anywhere, despite me often feeling like I did well in them. It took me a few months to realize that they were actually just hearing some murmurs about what my company was doing and wanted to learn more about it- we were approaching the problem in a unique way for the time.

Even seemingly innocuous details can give away a lot about a company's strategy. If you are working on say AR at Amazon, and you mention that your group is about 200 people, that says that they are not only going into this space, but doing a reasonably big push on it as well, which might be news to say FB. (This is completely contrived, I have no idea if Amazon is doing AR stuff at all).