This is related to the branching forms report:
A means of checking for the existence and uniqueness of a property is required when 1 or more properties of a page are inherently unique to the page, and checking them for uniqueness can prevent duplicate entries into a wiki.
For example, if a wiki were to be about objects that have unique serial numbers (cars, firearms, aerospace parts, social security numbers, national ID numbers, etc), the form for entering these objects into the wiki should check if the field for the serial number would be unique on the wiki. If it already exists, it should take the user to the form for editing the existing entry.
Right now, Semantic Forms can only do this with article titles, not with properties. Article titles can vary and be unpredictable, but the serial number is always the same, and ideal for uniqueness checking.
Another more detailed example would be a university using a wiki to keep records for the employees and students. Many students have the same name, but they all have unique student ID numbers. All employees have a unique social security number. Most students have unique social security numbers too, but some do not.
Since everyone on the campus will have at least one of a social security number, and/or a student ID number, but it is not known which ones they will have, it is necessary to create a special record ID instead of just using a social security number or student ID number as the wiki page title (page titles are harder to use in queries, also).
Each person on the campus could be automatically assigned a unique record ID to be used as the article title for their wiki page, including non-students who do not have a student ID.
Being able to check for uniqueness and existence of social security numbers and student ID numbers would help ensure that no person gets entered into the system twice, or enrolled under two different student ID numbers. It would also help in keeping records accurate for students who change their names, social security numbers, or student ID's because it is unlikely that all three of those pieces of information would change at the same time.