Because when I signal you suddenly notice that I’m about to pass you and you try to block me or steal my lane. If you’d just come to terms with being a slowpoke and let me on my way we’d be at peace.
When I’m in a turning lane that is marked for left only or right only I don’t signal because its redundant. All other times I do signal.
Sometimes I signal when I’m not about to turn. For example, if someone pulls up next to me at a light indicating he will try to cut me off on green, I will signal that I’m about to turn into him and rattle him, then blow smoke in his engine intake when the light turns.