I LOVE MEMES!
xD
The same way the spread of information originated. With advances in communication technology came a larger playing field for the spread of cultural information. The rise in popularity of the internet has taken it to a whole new level, hence the popularity and creation of the "internet meme". The reason why the internet has boosted this idea of memes, I think, is due to it being a largely interactive medium. Television, movies, music, newspapers, these are not as interactive, and thus, memes created within these mediums would mostly be created by the producers of them - which is a fairly small section of society. It's kind of like, "here you go people, here are your memes, these will be passed through culture until we create new memes for you". Where as with the internet, it's more like "we are the people, these are our memes."
They can express all sorts of things. For example, on 4chan, you'll find a lot of memes expressing an attitude that is sexist, racist, anti-government, anti-establishment, anti-political correctness, anti-conformity while being completely conforming.
They can express social norms, or they can express the bizarreness of social norms.
I think memes can give a sense of belonging. Or maybe people just engage in them for the lulz.