It’s actually used to make it a different term. If you tag something #weeds, that won’t show up in a search for weeds. But, if you’re trying to comment on the latest episode of the tv show Weeds, then you want to distinguish your post from someone talking about how their garden is infested with weeds. It’s also a way of saying ‘If I was filing this, this would be the label I’d put on it’. So for example, if I was bitching about my crazy mother, I might tag it #BadParents, or a post about my latest A grade on an exam #SuckOnItBitches (I always capitolize the first letter of a new word to make it easier to read, but most people don’t).
Charlie Sheen has declared that he is “winning”, when he signed up for a Twitter account, he got over 11K followers before his first tweet, and became the person to get 1 million followers the fastest, thus “winning” Twitter. I’m going to guess that #dealwithit means, deal with it, you people who are hating on Sheen, because he’s a freaking rock star from Mars.