The Expression evaluation tool (sometimes called the Watch tool) makes it possible to evaluate expressions while the application is stopped in the debugger. A typical use for it is the case in which you would like to know the result that an external feature would return. Since you do not have access to the C memory of the application, doing this would be impossible without the Expression evaluation tool. You might also want to know the result of an Eiffel feature whose body is too complex for you to evaluate.
If you keep the mouse cursor over the expression cell of line, you get a tooltip describing the expression result.
If the expression has an error, or if the evaluation raised an exception, you can obtain a more detailed description by double-clicking on the expression row.