Messaging policies are used to control which chat and channel messaging features are available to users in Microsoft Teams. For example, a business may not feel at ease with the Team using GIF's or Stickers in what they class as corporate communications (I.e. in Sales or Finance), whereas it may not want the Team to edit or delete posts for compliance reasons. A great thing about Teams is that it isn't just a one size fits all policy in terms of Messaging - sure, the organisation starts out with a global org wide policy, and this overarching policy can be amended to make changes to all users if required - but custom messaging policies can be implemented for specific users/teams too. Regardless of the debate as to whether aspects of messaging policies such as GIF's ought to be used, this flexibility gives organisations much needed choice and flexibility to how they want messaging in Teams to be used and by whom.
There may be an app your Team frequently uses. For the purpose of this article lets take this app to be Microsoft Flow where the Team is repeatedly adding Flows between apps such as Forms and SharePoint and Twitter and Powerapps across multiple Teams. They could always add Flow as a Tab to a particular Team, but with the increasing amount of Teams this wouldn't make much sense as the Team members would have to remember what Team, or - as is more likely - they are now adding Flow Tabs across multiple Teams which is littering Teams with Tabs which don't necessarily need to be there.