ability to handle increased demand/load/use
Ryan Tomayko (in 2005) on why LAMP systems scale better than complicated Three Tier architectures (that want a physically separate Application Server). More specifically, the Shared-Nothing variant of the Logical Three Tier Architecture says that the simplest and best way to build large web based systems that scale (and this includes enterprise systems goddamit) is to first bring the presentation and app code together into a single physical tier. This avoids remote object hell because the presentation code and the business logic / domain code are close to each other... the database is the only thing managing fine grained state because that's its job, and then throws up caching HTTP proxy server(s) (Caching Proxy Server) in a separate (and optional) top physical tier. Cached state is maintained on a much simpler, coarse grained level with relatively simple rules for invalidation and other issues.
collection of presentations on WebApp Scalability http://www.slideshare.net/group/webapps-scalability/slideshows