Awesome song, indeed, but how does it relate to the French revolution? The basic idea of the song is that someone who had lots of power for all the wrong reasons lost it and now feels empty. That may be attributed to any revolution basically - French or Russian, for example, or even the fall of the iron curtain. It may also apply to single people and, figuratively, even to losing your partner or love. After all, there's a line saying "sleep alone" in there.
The purpose? It's Coldplay. Their music is feel-good (not that I don't like it). Titling an album "Live the Life" is pretty in line with others of their song names like "Don't Panic", "We Never Change", "Everything's Not Lost", etc... and maybe that's not fair for me to say. Their first album is very feel-good, and the later ones are certainly more mature, but I think they've tried to hold on to that aspect of their music since their premiere album.
The Album cover that fetures Viva La Vida is a painting of the 1832 Revolution, so I furgured it was either about the French Revolution, or about Louis XVIII being forced from power after the Borbon Dynasty was restored.