This is a common feature in most debuggers, allowing the developer to "pin" variables for easier tracking and viewing. Marking a variable would pin it and keep the variable expanded while stepping through the debugger, because currently everytime you advance the debugger all variables collapse. This makes debugging variables inside of loops very frustrating and time consuming, since you have to constantly find and expand the variable you are trying to track.
This can be taken a step further by allowing for specific attributes of a variable to be watched, which would pin a limited expanded view of the variable with only the watched attributes making debugging specific attributes extremely easy and developer friendly.