Depends.
You can use poison soup to make Zuuna's Perogi (+3 energy form attack power, this can be huge in certain boss battles, for example, the dad golem in the sunken city can be killed in just one of his vulnerable cycles, instead of having to go through 3 cycles)
Swamp Cake adds poison damage to the otherwise harmless bubble attack of fish form.
Rainbow Soup is just for fun, try it once, and it should learn you to never do so again.
Poison Loaf doesn't do anything useful. Though if you want to damage yourself (for example, to test out if healing from spirit form absorb really works) then this will do it in a very controlled way.
