The new Yammer app in Teams lets you use Yammer without leaving Teams. And it looks better than the interface we have been using in Yammer for years now.
To begin use it, click the three dots on the left, then select Yammer. Not there, search for Community. The Yammer app for Windows will be replaced by a newer one in March/April 2021, and if you use it today make sure you get the update. You can select to use Yammer in Teams, Yammer as an app, or Yammer on the web.
Why use Yammer and not Teams?
Yammer can have more participants than Teams in a group. To have discussions with people in a group of more than 10.000 users, then Teams is not an option, but Yammer may be. Yammer do have support for conversations, but lack many features that Teams have.
You can also see Yammer as the outher ring where people that do not know each other meet in big organizations. When time to collaborate in a smaller project/group, then use Teams instead of Yammer.