Butt mode is the only one that changes the dialog. A majority of them are progression and starting with items and upgrades, but there are a handful that change up the game pretty drastically. Moon jumps, turning every surface into ice, making
Butt Butt Shovel Knight gigantic and changing his hitbox to compensate, being able to pogo on every surface (ducktales mode?)
The doc has descriptions of what most of the codes do. The ones that have nothing haven't been tested past the first few screens yet.