Maintenance of the interwiki map requires being able to find all existing pages which use a given interwiki prefix. The `iwlinks' table is very convenient for this. However, it only includes pages that have been edited or null-edited since the iwlinks table was added, i.e. when MediaWiki 1.17 was installed. (Purging, by contrast, updates the page_touched date but does not appear to update the iwlinks table.)
See the following three database queries for confirmation of the above:
https://quarry.wmflabs.org/query/35250
https://quarry.wmflabs.org/query/35265
https://quarry.wmflabs.org/query/35266
This means that a lot of old pages are potentially missed in searches of the iwlinks table. See prior discussion here:
https://meta.wikimedia.org/w/index.php?title=Talk:Interwiki_map/Archives/2014#Can_the_iwlinks_database_table_be_trusted%3F
At the time of the discussion, @MZMcBride suggested instead null-editing every page. However, that seems like a Herculean task compared to running a maintenance script, and nobody ever got around to doing it.