First, the "locution prépositive" auprès de can be used for introducing a complement designating a person having official duties, or some institution or entity, and showcases, aside from location, this idea of an assignment and being accredited to be there (translated quickly from the TLFi, auprès (de) I A 3. examples 32-25). Outside of this, auprès de is generally often about close and/or continuous proximity, and considering the duties of the auxiliaire juridique and the very word auxiliaire, that seems also fitting in this case. Then consider you might even find près without the preposition de with that sort of legal phrasing with the same meaning:

— Dans la langue juridique et diplomatique : L’ambassadeur de France
AUPRÈS DU Saint-Siège (Ac. 2000), ou, sans de :

[…]

Dans la langue juridique et diplomatique, près sans de remplace
souvent auprès de : Expert PRÈS le tribunal de… — Notre
ambassadeur PRÈS le Saint-Siège (MONTHERL., Treizième César, p. 49)

[ Le bon usage, Grevisse et Goosse, éd. Duculot, 14^e,
amalgame des § 1046, 1073 ]

This is more legal/diplomacy related usage from what I can tell, not necessarily casual prepositional affinity, even though it’s in line with the idea of close proximity which auprès de is all about. If it wasn’t for this usage, I could see the preposition de or pour here like you suggested, but not à imho; I could see à with the noun or something like "rôle d’auxiliaire (à) "as opposed to the title (clerk/auxiliaire) per se, but I couldn’t tell you why exactly, maybe some prepositional preference depending on whether it’s a person or not.