it's not for understanding magic, its for communicating about it in Official Capacities, usually with other wizards, everybody can use the Arcana skill but that doesn't mean everybody understands a language, so it serves the same purpose as any other language, some things you can't even attempt to understand if you don't first understand the language they're written in, your ranger might have expertise in nature but they're not going to be reading that book on dragons if they don't read draconic, or at least, they're going to be attempting to read it at an incredible disadvantage.
edit: if i hand you a science book in a language you can't read and say 'don't worry, with a high enough science skill you'll understand all of this' that doesn't mean it's not going to be infinitely easier process if i hand you the same book written in a language you do understand.