At first I thought the purpose was to spare sensitive individuals from seeing dead animals by being able to filter them out. However, that wouldn’t work very well since most observations are not annotated. I guess they could filter for living species, but I don’t think they’d get very much.
Then I guessed it would help some forms of research, but I’m not sure that it would work so well for that either! You couldn’t tell if there were an increase in deaths necessarily, and you probably couldn’t determine means of death. And there would be lots of confusion with the above-mentioned problem. Now that you mention it, I think I might stay away from that button unless the observation is my own.