Yammer was mainly a communication site my company used to provide information about internal events.
The most valuable feature of Yammer is that it provides integration with Teams. You have a side-channel method of receiving those other than just email. Sometimes, when email is the only way something arrives, it gets lost in the shuffle.
Yammer still comes via email, and I look at it when I see it. However, it tends to be less easily dismissed when it pops up in Teams. I'm with a different company, and the company that I was getting it from doesn't go cross-channel into different teams. Seeing it in Teams was helpful when I was at that company.
I like the fact that you can comment on Yammer posts, interact with them, and see other people interacting with them. I think that's valuable. It's similar to Facebook, but it's for business.