Sometimes authenticated clients can abuse the system intentionally, depending on what you're doing with it.
You will likely discover at some point that your service has to interact with other services, which may or may not be easily doable with a particular client orchestrating all of the work itself.
Depending on your problem domain, you may need to do things on behalf of a client when that client is not connected to the service/the Internet/etc. This counsels having a non-dumb server side piece available.
There exist many other reasons one could name. What are you thinking of doing in your dissertation?