Most times I use lol as a means to convey my message/text/whatever when it's meant to be humorous because these are things that cannot really be understood by someone else when they read it and it could be taken the wrong way. And on the receiving end I also understand that people aren't actually busting out laughing when they reply lol.
Just as an example say you send me a message with something you did and I reply with either:
You're the worst.
You're the worst. lol
It's sent as a joke with both but with the first one you would probably take it wrong because you can't see how im saying it. Know what im sayin?