Is the main value to the expression debugger that it makes explicit the order of evaluation, or the specific intermediate values? (or is it pretty much both that provide the value?)

It makes explicit stuff like operator precedence, but it's especially helpful when you have to explain function calls and recursion.

For the latter specifically, Thonny has a mode where for every call stack frame, it opens up a separate child editor window that displays the code of the function in that frame, highlighting the current line. When you step through, the visual substitution of expression results happens in the window for the frame where it is happening. Thus, when you have a chain of recursive calls, you can see all the substitutions that it took to get there.