in some cases, animals are the best way to find out if the effects of a treatment are statistically significant. with humans, they may not get enough volunteers to make a proper sample size. also, there are lurking variables that may influence the outcome of a study that can be controlled in animal populations but not humans. for example, they use animals from the same litters in the same test groups so that they all have similar genotypes. this way, one is not more likely to succeed because of genetics than an other. you cant say to a woman 'we want you to have 50 children with the same man in the next two years and then hand them over to us' just doesnt work.
infact, thats what they had to do to prove smoking causes cancer. even though they could see that smokers had a higher rate of lung cancer, the tobacco industry would argue that people who were more likely to develop cancer could be more likely to choose to smoke. and you cant take 1000 nonsmokers and say 'this half is going to smoke, this half isnt' so they used beagles. i thought it sounded ridiculous, but ive grown up knowing that smoking causes cancer, they werent sure at that time. they were just trying to do the most unbiased study possible for the public.
some animals are also used to produce pharmaceuticals that we rely on to live. the antibodies found in most vaccines cant be manufactured so animals are injected with dilute solutions of the virus and their antibodies are harvested. is this animal cruelty? they are living in labs and repeatedly poked with a needle, but how many diseases have been wiped out because vaccines are first tested on, and then produced by animals? an other one is antivenom. it is admittedly a hard and stressful procedure on the snake to be milked, and again horses and goats are injected with it to harvest antibodies. but what happens to you from a snake bite, depending on the venom, is some of the worst things i have ever seen. the perfection and production of these anibodies, through animal research, has saved thousands of lives.
as much as we all hope this might stop one day, there really isnt a good way to test drugs, treatments or therapies without a living body.