What might you want to use this for? Well, if the “linking ISSN” is deployed succesfullly, and the information is successfully included in the information available from the ‘issn portal’, then this is a machine-actionable source of correspondences between ISSNs that really represent the same title in different formats. I trust that many of my readers can think of all sorts of uses they could make being able to embed that information in their various discovery applications.
OCLC xISSN also can potentially provide some of this data in machine actionable form. (Haven’t explored it yet myself). I assume that xISSN correspondences are currently algorithmically/heuristically generated from what information is available in a cataloging record, as opposed to the “linking ISSN” based metadata, which presumably will be manually controlled? But then an interesting question is the cost comparison of these two services licensed for the uses we’d want to put them to. Would be nice to have two competing metadata web services available for a change, instead of usually having NONE that do what we need.