Reprioritization often leads to partially finished work items. Those work items have just a small chance of being prioritized again. Even if they do get reprioritized, the product or the code base will be further along, which means the partially finished work item need to be redone or even rewritten from scratch.
All of this does not come for free. Sometimes there is lack of understanding from C-Level management as to what reprioritization actually costs. The intention of the burn money chart is to assign $$ amounts to partially finished work items to make the wasted resources visible to stakeholders.
Bottom line, having the amount of sunk costs in a chart, similar to a burn up chart, helps to get momentum at C-Level Mgmt, when it comes down to the initial question of the monetary benefit of transitioning to agile methods.