my friend just got one. the birdie bag works the same as dirt, chalk, resin, etc. the palms of my hands never sweat so i never see a use for one really except for when it's so humid out that if i touch any other part of my body i get sweat on my hand.
my friend still prefers dirt to the birdie bag, but dirt isn't always available. the birdie bag has an advantage over chalk and resin because they tend to cake on the disc and in the creases of the hand while the birdie bag dries things out and excess will fall away. it will make the disc feel dusty, but better than a sweaty grip.
you can get a small bag for like $4, so it's probably worth a try if you have ever even considered one.