Applications need to constantly evolve to keep up with an evolving world. When Gmail started, it was just a simple mailing utility and now it has evolved into much more than that. These frequent changes demand frequent deployments in such a way that the end user doesn't even know that a new version is being released. By dividing into smaller units, a team can thus handle frequent deployments with testing and get the feature into customers hands quickly. There should be graceful degradation, that is, fail fast and get it over with.