That argument speaks for itself. The idea is to reduce the suffering caused by thoughtless cruelty. I don't know if plants feel or suffer from pain, but I do know animals suffer. I know people suffer. And even t hough a human or animal might not cry out in pain, I will still not stick them with a knife simply because I didn't get a response.
People kill for fun, for sport, for fashion, for power and for wealth.
We often torture before we kill. We conduct wholesal e slaughter, from human genocide to factory farming. From zoos to circuses, rodeos to hunting, vivisection to trophies, people exploit the hell out of animals. It's also true, people exploit each other with the same insensitivity and lack of remorse.