Some Wikidata properties are expected to hold the same values when they are both present on the same item. This is the case for https://www.wikidata.org/wiki/Property:P4285 and https://www.wikidata.org/wiki/Property:P269 for instance (see https://www.wikidata.org/wiki/Q334065#P4285 and https://www.wikidata.org/wiki/Q334065#P269 for an example). These two properties are used to link to different databases which use a shared identifier.
The question of whether this is the desired way to link to two databases which share a common identifier is still being debated (see https://www.wikidata.org/wiki/Wikidata:Property_proposal/Directory_of_Open_Access_Journals_ID and https://www.wikidata.org/wiki/Wikidata:Property_proposal/Plants_of_the_World_online for instance).
The issue with this approach is that it creates some redundancy. One possible way to help with that redundancy would be to have a constraint that would enforce equality of these values on a given item. This constraint would be useful for existing properties which have this redundancy issue and potentially for others if we decide to use this solution in other cases.