I would find another vet immediately. My vet's rule of thumb is if one of my cats throws up once, not to worry but if s/he throws up again, I should call him. Your cat may have something as simple as food allergies but it could be something more serious and time may not be on your side, if it's a more serious illness. Tuna is ok to get a cat over the hump and eating again but it's human food and doesn't have all the nutrients that a cat needs, especially the amino acid taurine; otherwise a cat can go blind w/out it. Please find another vet. For your cat to vomit every day is absolutely unacceptable. Imagine how miserable you would be if it was you retching every day. And if he drinks a lot of water, he could be diabetic or a host of other things. To answer your question, a steroid can be administered for inflammation, allergies, etc., a variety of reasons. Didn't you ask your vet why he was giving your cat the shot? Go to another vet, ask a lot of questions and if money is a concern, usually vets will accept payments if you make arrangements before treatment. Get going! And please update us. I hope your kitty is going to be alright.