So that users can only access so many entities during a page parse. This should apply to both Lua and the parser function whenever one of them needs to load a new entity.
This is needed to make sure that users can't load so many entities that things time out while rendering or even worse result in a php fatal. Also this could be needed as an emergency switch in case arbitrary access causes trouble to network traffic or the external storage.
An alternative approache would be to limit the overall size of entities loaded (this has the downsides that a page that rendered fine at some point might start showing errors at some point because data was added to entities on Wikidata). Another alternative approach would be to just mark all such function calls as expensive, but that's not very fine grained and wont allow us to react to eventual imminent problems in production (https://www.mediawiki.org/wiki/Manual:$wgExpensiveParserFunctionLimit). Or we could just not prune entities from the in process lua cache and let Lua run out of memory if the users are using to much (that's not very fine grained and interferes with other modules).