It's used to discourage effort and to minimize the suffering of non-human animals by trivializing it: "even plants feel pain, pain is everywhere, there is no point thinking about reducing pain and suffering, it's futile."

Such people usually have some other criteria of "who matters", which ignores suffering. Usually, it's a hierarchy.

The same hierarchy extends, at upper levels, to human intraspecies hierarchies, which is how you realize that the AI bro TESCREALists are fascists, racists, eugenicists and so on. They mention sentience in passing, and then skip to "intelligence", a much more vague concept, but something that they love to track ... as in the tradition of IQ and race science.