Having external reconnaissance on your establishment can be a life saver if you have an un- managed server left on the net. This can leave an attacker leverage to creep deeper into your network, collecting data and credentials along the way.
Footprinting establishes a clear picture of an organization that allows an attacker a full map to figure where attacks are easiest. This is your front lines per say. This is the information that is leveraged on the digital exploits to be used after this process. Having an idea of all your firewall policies and not haphazardly leaving leaked credentials for an intruder to break through further.
Anything is valuable information for exploitation from web pages, organization associations, details of your locations, employee info to archived information.
Some of the scariest ideas is the fact that some public servers have to hold your network information for routing users to any of your available services. This means people can do research on you without alerting any of your services of any attack.
Footprinting and mapping can be your first and last line of defense when it comes to your security stance and policies. Preventative maintenance can save your hide by catch servers not updated, long password expiration, or valuable credentials being served up servers to be snatched up. This is what security is all about.