It don't really matter if you take this day seriously or not. Google does.
Over the years, Google has built a rich tradition of pulling a fast one on the world - including sending us to the moon, making us smarter, reading our minds (although some might say that's not really a hoax) and helping us find that special someone. There's a whole host of others as well.
The impression most people (including myself) people have of Google is that it's a fun place to work. This certainly reinforces that image. There's a heightened sense of curiosity around this time every year - what will Google come up with this time? And there's a nice feel-good factor to the whole thing.
Inexpensive, fun marketing.
A concept worth emulating.
But hey, it's not always fun and games. Sometimes, they sell you a double dummy.