Many people will attempt to poison the rats, and you probably will notice the poison on the shelves right next to the traps. While the poison may successfully kill the rodent, you may not achieve the full extent of what you want.
Rats are highly intelligent and observant, so they have adapted to people poisoning them. They will often eat tiny bits of food from the edges to sample it, this is also a test for poison. This way, even if they are poisoned they only ingest a small amount which will often only make them sick. The rats will then spread the word around that there is poison food, and they will be much more cautious.
However, let’s say you do manage to poison rats enough to kill them. You may think this is exactly what you want, but if you’ve ever had a pet pass away you may know that most animals seek a dark secluded place to pass away in. Rats will do this as well, and may end up finding a spot where you cannot reach - or worse, falling down behind a wall. You are then left with a decomposing rat in your home that you are unable to locate. Whereas, if you use snap traps you are able to locate the carcass before the odor begins.