Lmao is often used to make something sound less depressing. Or if you don't know what to respond with.
Person 1 : *Types message* "Oh man, that sounds abit depressing. Better add a Lmao at the end to mix them up."
Human 1 : "Did you know 175 humans eat atleast 2 scorpions in there sleep?"
Human 2 : "Lmao you crazy bruv."