I've always enjoyed public art, from mosaics in subway stations to murals in parks. It lends spaces a sense of personality, shaped by the people who interact with the architecture around them as much as by the planners and architects who first conceived them. It makes a place look truly inhabited and a structure look alive, more so than a form designed purely for unobtrusive function would otherwise be.
Of course, there's often a more specific message behind public art -- the very nature of the beast lends itself to political and social expression. One might expect that public art in fraught environments would be more message-driven (even though the risk to artists is correspondingly higher). One often forgets that beauty, with no ulterior motive, can be a message in itself:
http://www.policymic.com/articles/90589/an-iranian-artist-is-beautifying-tehran-with-surreal-street-art
I hope this artist continues to create and inspire.