To channel Biggie Smalls via Ben Horowitz, "don't you know Bad Boys move in silence and violence?". If competitors don't know you're well funded, they're more likely to ignore you. On a more practical level, it keeps the bottom feeder companies which troll crunchbase/techcrunch/etc and spam you with recruiting/etc. offers for months away. There's not a lot of upside in letting people know you're funded before a product is launched.