Okay, maybe not literally; but consider the following. No serious musician would want to admit that he couldn't just play Happy Birthday off the top of his head; but suffice it to say, that has been known to happen. Even a three chord song can thwart you if it catches you off guard; and if you've ever had to pick up the devastated heap of a musician wrecked by the inability to anticipate that pesky seventh on the first chord of the third phrase (What the heck is that, anyway? A secondary dominant? Short little "easy" songs aren't supposed to have those!), then you might want to invest in some Happy Birthday sheet music. Think of it as an insurance policy.
For the player of any skill level who doesn't hold himself to that level of expectation for memorization or picking up a song by ear, Happy Birthday sheet music can also be quite helpful. It isn't just pianists who might want to accompany a group rendition of the song, but rather anyone on any instrument might want to play along, or perform alone. You are likely to be able to find versions of Happy Birthday for any instrument from alto sax to tuba, and in many cases, duets with piano accompaniment as well.
You may be able to find some really fun digital sheet music arrangements of Happy Birthday. While some will be the bare bones chords and melody so that anyone can easily sight-read, other arrangements can be quirky, jazzy, or in a classical or pop idiom. You haven't explored the full potential of Happy Birthday until you've browsed some of the versions that you can find online.