I can't claim to know all of the reasons, but the main ones that I use it for is for a combination of Data Security and Performance.
We have a data warehouse set up. This contains many more data that you can actually bring into Power BI (it errors if you try). Making use of the tabular model allows us to shortlist the data we wish to include in our project. We are also able to front load a bunch of calculations onto the server whenever it refreshes, rather than overloading the Power BI side.
Lastly, we have users in the business that will want access to sensitive data. We do wish for them to have some amount of 'self-service' but we cannot allow them access to literally everything. So we provide them a sanctioned version, then they are able to do whatever they need to do from within the small box of data we have given them access to.
