No secret, actually, it makes sense as soon as you see it. The Burp Catcher is a burping cloth with a folded pocket on the bottom, which catches burps.
Now you can get all Beavis & Butthead about it and say, "Burp Catcher? Why isn't it called a Puke Catcher, dude?" But you have to remember that this is a prime gift item, and unless it's Greek Week, something called a Puke Catcher would probably make a less than scintillating gift.
And besides, before someone actually has a baby, they may really be in the dark--whether it's through happy ignorance or desperate denial doesn't matter--about just how much of their mental bandwidth will be consumed by things like poo, pee, and puke.
So while a Burp Catcher helps soften them up for the inevitability ["Oh, thank you! Whatever is this little trough for, I wonder?"], it will also actually protect things you wouldn't want to see puked on several times a day--things like clothes, carpets, furniture, hand-blocked wallpaper, and cats.