I’ve been reading Heikki Mattila’s book Comparative Legal Linguistics (in Finnish). This thorough work reveals the close relationship between contemporary legal texts and Roman law. During the Middle Ages, lawyers in Europe used only Latin, which was also the language of law and adminstration.
In addition to its practical duties, legal language also aims to uphold the authority of the law. For this reason, legal documents stick to formal and even archaic expressions. At the turn of the millennium, Finnish-language legal texts contained more than 600 Latin phrases, such as ex ante (from before), in casu (in the case), expressis verbis (expressly). In English these are even more common; every year the U.S. Supreme Court receives hundreds of petitions for a writ of certiorari (to be informed).
Lawyers will often use foreign words and abbreviations bona fide (in good faith), both out of practicality and out of habit. But in some cases, they may act mala fide (in bad faith), using specialized terminology to increase their own stature or to keep ordinary mortals from engaging in legal business.