When creating a string attribute, one can filter against one value of that attribute. But one can't filter on multiple values such that all the requests that correspond to any of these values are returned.
Why that may be useful:
- sorting on continents when the attribute describes the countries or, more generally, on super-categories when the attribute stores the category.
- this would allow using a string attribute to store information where each company may have more than one value that's correct (e.g. they have both a type1 license and a type3 license). Then you would filter on all attribute values that include type1.
This is even more frustrating when one realises that tags and status allow exactly that...