Grandparent comment originally said, "But how are they going to repay that investment?". LOL, investors don't want repayment, they're gambling for a shot at a dump truck full of money. If they don't get that, thanks for playing the VC lottery, take care, bye.
Which begs the question, how is The Browser Company going to live up to the 550 million dollar valuation? I'm not fluent in VC-speak, but I think the math is something like this: you need $R in revenue annually to be valued at $V, where the difference between R and V is some multiple M, usually 10-50ish. Let's say 50, so R = $550/50 = $11 million/yr in revenue or a more conservative 20 multiple, R = $550/20 = $27.5 million/yr in revenue.
If they charge $10/month, then they need ~91k users, that gets them to $11 million in revenue from users directly. If they need to hit 27.5 million, then they need ~229k users paying $10 month.
Not the worst investment after you do the math, maybe it's theoretically a sustainable business.
But startups don't raise money at these numbers to become sustainable ten-million dollar-per-year businesses. Investors want that unicorn status most of the time. At that point, I don't know if the math makes sense because to be a unicorn you need like $50-100+ million in revenue per year. That's easily 3x or more users, you're approaching 500k-1 million monthly active users.
That's a lot of users for something Apple, Samsung, Mozilla and Google give away for free on all OSs already.
Edit: I just looked up their funding raising information. 137 million raised, and they're series A/B'ish right now. That's a lot of money, they definitely have big ambitions.