Lets say you have a multi-value field called "Fruit" with the following options:
Lets say you want to add "Orange" to a set of contacts. If you use the batch updater to make an update to this field, you can only copy all fruit options from the first contact to all contacts. In doing so you overwrite the pre-existing values for "Apple" and "Banana" that the other contacts had previously.
Instead what we need is the ability to add or remove a single option from all contacts without affecting the other options.