Why do you need to understand BGP?
When BGP is configured incorrectly, it can cause massive availability and security problems, as Google discovered in 2008 when its YouTube service became unreachable to large portions of the Internet. What happened was that, in an effort to ban YouTube in its home country, Pakistan Telecom used BGP to route YouTube's address block into a black hole. But, in what is believed to have been an accident, this routing information somehow got transmitted to Pakistan Telecom's Hong Kong ISP and from there got propagated to the rest of the world. The end result was that most of YouTube's traffic ended up in a black hole in Pakistan.
More sinisterly, 2003 saw a number of BGP hijack attacks, where modified BGP route information allowed unknown attackers to redirect large blocks of traffic so that it travelled via routers in Belarus or Iceland before it was transmitted on to its intended destination.