Once implemented, Schema:ExternalLinksChange will provide a large dataset on editor linking and referencing behaviour. This can be pulled together into a tool for use by the Wikipedia community as well as The Wikipedia Library.
The community would primarily find this useful for processes such as anti-spam efforts; being able to see a full log of link additions is of considerable use when tracking down sockpuppet accounts spamming links, not just for being able to go straight to the spamming user, but also for seeing link additions that have already been reverted (Special:LinkSearch only shows existing links).
As part of the access donation program with TWL we currently send partners quarterly metrics updates detailing the total number of external links to their resources on Wikipedia. The aim is to show how many links have been added as a result of the TWL partnership and thus demonstrate its success (or failure!). However this process is imperfect in many ways. The primary drawback is that we rely on Special:LinkSearch, which shows the total number of links as added by all users, and only on a specific language Wiki. We therefore have to infer whether a set of resources is worth continuing; while this can be relatively easy if the partner had very few links present to begin with (we can assume the vast majority of any growth is because of TWL), for larger partners the TWL additions are dwarfed by the overall noise of standard non-TWL user citations. This schema will help us narrow down which partnerships are successful, where links are being added to partner resources, and what proportion of editors are actually making use of their access on Wikipedia, among plenty of other interesting data points and trends.
These two cases share requirements for most of the same data and processes, though there will be some additional requirements for each, primarily in how the data is presented and filtered.