There maybe several reasons for this, like legacy applications or just their process. Usual scenarios for having to use multiple accounts are: customers or partners are creating an account for you in their organization, instead of inviting you to their tenant as guest. However, it is not the real world to everyone: so here is a Survival Guide that will make it a bit easier. And I agree – using Teams with just one account is the most simple use case. Usually you have your homebase (home tenant) where you have your work account. One very often heard challenge with Teams is that it is difficult to handle work when you have to use multiple identities (accounts).
