It's just a thing we Do, and it's fun. But it's really, really terrible for the environment.
When a balloon ascends into the heavens, it doesn't end up on Jupiter.
That colorful little scrap of latex may end up living in the Great Pacific Garbage Patch. It could get tangled up in the flippers of a sea turtle or be eaten by one who mistakes it for a jellyfish.
Or a bighorn sheep could mistake it for forage, or it might land in some farmer's pasture, where a cow chokes on the string.
According to the U.S. Fish and Wildlife Service, animals of the land, sea and air are equally susceptible to mistaking deflated balloons for food, or, arguably worse, getting tangled up and strangled by the ribbon attached to it.
And it's true, some balloons do break down eventually. A big part of the reason releasing balloons is permitted in so many places is that latex balloons are technically biodegradable -- it takes one between six months and four years to break down completely, though they deteriorate in seawater more slowly than they do on land.
Mylar balloons, on the other hand, are made out of NASA-grade nylon and are not biodegradable, so they can hang out in the environment indefinitely.
Helium balloons can interfere with airplanes. In Singapore, you have to get permission from a government aviation regulation agency in order to release balloons.