I came across this article that actually gives some good reasons for the benefits of learning another language other than vague promises of discovering new semantic frontiers. (Which may happen here and there, but it’s only subtle, the difference between saying gabion and wire baskets filled with rock and stone.) This is, of course, aside from the obvious functional/pragmatic benefit of needing to communicate with someone. These are finally good reasons in the realm of geographically isolated, monolingual Americans doing it as a hobby.
The first one is the weakest reason, but I like how he says “communicate more clearly” instead of “use proper grammar”. Not sure if it does that, but at least he avoided sounding prescriptivist. Self-awareness of your native language is really only marginally useful, though it is awfully entertaining. Not knowing the linguistics of your own language won’t prevent you from communicating or creating art with it.
The rest are about studies that have found cognitive benefits to bilingualism. I haven’t looked at them, and this is Huffington Post, but the summaries seem sound.
And all around more substantive than the usual pontificating on foreign tongues and other Whorfian things.