Brex Lands $2.6B Valuation with Latest Funding Round

Great news. I think Brex has a great idea. But if anyone can, please do shed some light for me on how they grew so fast ?

I mean, I can see how Bird etc would achieve a 2B valuation fast, they're B2C so you can see them everywhere.

Are they signing up thousands of customers at an astonishing rate or is this funding more related to potential growth ?

I don't understand Brex's USP. Amex gives corporate credit cards. What does Brex do differently?