Hi Dario, "for what it's worth " has a slightly flippant connotation. It means, for lack of a better explanation, "take this information and do what you like with it". "For what it's worth, this cereal is better." The speaker doesn't want to say "This cereal is better". "In my opinion" is another way to say it. It is used to soften a statement, such as the one you used. "Or for what it's worth, why did you come back?" The speaker is trying to convey a feeling of not caring whether the person came back or not. He may care very much, but he doesn't want the person to know his true feelings. I hope this helps.

Hi dario g, Just my opinion as for "for what it's worth" the idea seems to play something down, to convey the idea that the message is not so important, not necessarily accurate (it is just your opinion after all), or to try to fool someone into thinking it doesn't matter to you. I suppose it must be useful when you wan't to give someone a piece of advice and don't want him/her to think that you might be hurt if your advice isn't followed.