If you want to use a report widget to find and display the count of unique values, you cannot do this if your field is a picklist.
For example, if you are reporting on timeline entries, and you have a picklist on them and want to answer “in this subset of timeline entries, how many different picklist values from [field] were used?” you would not be able to do that. Your picklist would need to be converted into a string field, which would involve either doing the whole thing in Data Designer, using a rule to convert your picklist to string data in another field, or not using picklists to begin with.
If you do end up using a string field instead of a picklist, regardless of the method you use to get there, you lose the functionality of a picklist, especially in filters.
Just ran into this need today. I wanted to count up the number of field values used. Count Distinct is not an option.
It would also be useful to enable count unique/distinct values for Boolean fields.
I was just going to add that I noticed in Rules Engine I can use Count_Distinct on a boolean field. It is only in Reports that this option is not available.