While debates about the value of street art arise regularly, one of the distractions is perhaps the appearance of graffiti on buildings. While this is not street art in the same way as paintings or images that would usually be deemed "art", its symbolism and meaning may not be that different.
Graffitists and street artists may share many things in common: the need to leave their mark, to let others know they have been there, to send a message to passersby, to mark their territory. The hidden meanings behind graffiti might be just as mysterious as some of the hidden meanings behind artwork in a more traditional form.
Without knowing the person who left the graffiti or art work we won't know exactly what the purpose was. The graffiti may or may not be words that make sense to us, the outsider. Even if they are words that we recognize we may not truly understand the purpose of leaving those words there.
We could compare graffiti and street art to the images that we find from centuries ago of art work in caves and buildings such as at Pompeii. Those images give us many clues about the world that existed at the time. What was important to the residents, what they saw, how they understood their world - all can come through in their images. Of course we put our own modern day interpretation on those images to make sense of them the only way we can - through our own eyes, our own culture, our own experiences, our own contemporary understandings of the world.
Similarly we can only look at street art or graffiti in the same way. We could see the street artist or graffitist as representing a different culture, a subculture if you like. Therefore our interpretation or understanding of what they draw or write will be through our own lens - undoubtedly with our own judgments and interpretations placed on it. This can be why there is so much criticism of graffitists in particular. Understanding the behavior through a cultural and historical lens might help us to see it differently.