Can you explain why you think Robinhood is unrelated to finance and is jumping into it just because it's easy to?
Is it common for boutique brokerages to offer banking?
They have over 6 million accounts, which is more than E*TRADE and within a factor of 2 of Charles Schwab or TD Ameritrade. And these brokerages apparently felt enough of a competitive threat that they all adopted Robinhood's commission-free model.
Regardless, even if you don't consider Robinhood to be a major brokerage, they clearly aspire to be one, so offering banking services makes perfect sense.