If it's on exchanges, then it has nothing to do with high net worth clients, or any other kind of client, because they won't be trading with their clients specifically, they'll be trading with other market participants.

if it's OTC, then it could be part of a package of services for clients. That would still a bit weird, though, because a bank would normally act as a broker rather than a dealer for its clients - going to the market and finding them the best price, rather than making its own price.

I think that their reason is much simpler: they think they can make money doing it. I guess you could call that FOMO. But basically, they think that with their trading instincts, technology, and famously huge brains, they can move their prices around so that they can buy low and sell high without getting, as we say in the trade, their faces ripped off.