It's not necessary if it's not a destructive act that would immediately modify an entities properties.
What's your use case and the context? Are you editing an object where you're selecting attributes to modify, or is this a filter mechanism?
Editing entities: multiselect as part of a larger form
If you're editing properties as part of a larger form, chances are there's a final Save button, where users will first review the changes for any properties or settings modified.
These comboboxes are often part of a form, not the point of submittal itself.
Multiselect when used as filters
If you're using these as filters, often with data visualizations, you'll see the chart redraw. This is just a view setting that can be reverted at any point. It's a non destructive action.
In this example from Google Flights, the filters update the results. It's a long list, and while there's a
x, there's no need to submit for results:
Large selection lists: indicating selection status
If you have cases where you have long lists with many of the items out of view, you can subtly indicate in the header how many items are currently selected:
You also might consider reducing the tiger striping or test if you need it at all. I have not personally come across tiger striping in menus (that's just my experience, but maybe use a much more subtle tone if need be).