Question

Why do we cook our food and put lime on fish / shrimp to make ceviche​

Answer 1

So it could add a little more taste, it dissolves with salt

Answer 2

The reason is because ceviche (sev-ee-chay) is still very much raw fish or seafood.

The citrus bath the seafood sits in does turn the meat opaque, giving it the appearance of being cooked, and it does kill some of the many wee beasties present in raw foods that can make you sick, notably the toxin vibrio.