It's okay to eat farm animals because we breed them for this purpose.

This argument is saying that it is morally right to kill animals purely because we have an intention or imposed purpose for them. It is also implying that if we have a purpose for them, we can’t change it and in fact, aren’t even required to if it is morally wrong.

Now for this argument to be true it must extend to other species and situations i.e. humans. For example, throughout history people were often bred to be slaves with many never knowing any other kind of existence. Is this morally right because others in a position of power intended another being to be bred as a slave? Of course not. Something immoral doesn’t become moral based on another person's intended purpose for that being. If we decided tomorrow that the purpose of people with severe intellectual disabilities was to provide organs for the rest of society, would this be right? No. The purpose was something we imposed with no thought of morality.

Now some will say that humans ‘created’ many of these animals (i.e. pets and farm animals) thereby justifying their usage. However, selective breeding doesn't equate to creation. If someone did the same with humans, overtime people could look quite different but they would still be humans with the same rights as you and I. But despite this, whether we created them or not, it does not justify their usage. If we were truly able to create a human we could not morally treat them as we currently do animals. What is important is that they are still sentient beings who can suffer and therefore have a right to live.

At this point most omnivores will say 'It’s different because where human and they’re animals' but this is a completely separate argument based on speciesism and requires a separate debate, not one about purpose or who created what.

It is obviously clear that just because we selectively breed a living being (which is immoral) it does not give us the right to eat them nor are we justified in doing so just because we have invented a purpose for them that is completely imposed.