Bug Out Bag for Surviving a Disaster. Survival Mode Gear for the End of the World.

What is a "bug out bag"? Who needs one? I stumbled upon a web site that was basically about survival during "all hell breaking loose". The main theme was about having a bug out bag that had the essentials to help you survive the first 72 hours of a major disaster.
The assumption is that the world as we know it would cease to exist and everyone would be out for themselves. Help in the form of police and military would not be able to respond to everyone. You need to be prepared and the bug out bag would help. The bag is basically a back pack filled with items such as batteries, water purifier, matches, knives, multi-purpose tools, etc. The bug out bag would have to only carry the utmost important items so as to keep it as light as possible. The term bug-out means that you need to leave right now--no time to think about what to grab. I don't have one but maybe it has some validity (without sounding too paranoid). What would happen if a major earthquake or an asteroid hit were to happen right NOW? How about a nuclear meltdown accident? Do you need to buy some anti-radiation pills? Maybe a major terrorism event. When you need to get out of Dodge in a hurry, a prepared bag of essentials is literally a life saver. The boy scout motto is "Be Prepared", maybe they know something we don't.