If you need to visualize the difference from one measure to another you can use this field. For example, if you have Actual (AC) and Previous Year (PY) as measures in your data, you can add Previous Year into this field and Actual into the Values field.
If you fill this field, the waterfall chart type automatically is in delta display. Running totals cannot be chosen.
You can add multiple measures into the fields. The delta is then calculated between the sum of all measures of each respective field.
If you switch to stacked bar chart, the values from Values From build a second column.