When using the multi select custom field with a lot of entries (over 100 say) it would be useful if at the point of selection you could click on the field and start typing the name of the entry you desire and the multiselect list filters down based on the typed characters.
You can do this on the single select custom field and it would be useful if this was a feature on the multi select too.
As you can imagine, the time taken to scroll through a large list to find the correct selection could be eliminated via using the same typing style searching, the issue is worse when needing to select two options that may appear at different points in the list.
My use case is for helping filtering tasks by a particular entry from within the project rather than using the advanced search.