My guess: they're gearing up for an IPO. Maybe not for a couple years, but they see their market size as being big enough to go public, and they want a prominent financial backer in their corner when they do. That's the usual reason why profitable companies take investment. Microsoft famously eschewed all venture funding until a mezzanine round right before they went public, which was a.) so they could vet their investment bank and develop a relationship with them before it really mattered and b.) so said investment bank would have an incentive to hype up the stock and get the highest possible price at IPO time.