I can only imagine of sending a value from the server to the client which is (unchanged) sent back to maintain a kind of a state.
Precisely. In fact, it's still being used for this purpose today because HTTP as we know it today is still, at least fundamentally, a stateless protocol.
This use case was actually first described in HTML 3.2 (I'm surprised HTML 2.0 didn't include such a description):
These fields should not be rendered and provide a means for servers to store state information with a form. This will be passed back to the server when the form is submitted, using the name/value pair defined by the corresponding attributes. This is a work around for the statelessness of HTTP. Another approach is to use HTTP "Cookies".
<input type=hidden name=customerid value="c2415-345-8563">