I use it to generate a detailed resume and have up to date contact info for references. It also is how I keep in touch with some former colleagues that I was never friends with but still care about on a human level. They are not people I would ever have invited to be friends on Facebook, not to mention I deleted my Facebook account, but I still like to know where they are working at and catch up with them from time to time to chat about the industry.
There isn't a month that goes by where a recruiter doesn't try to get me to add them to my LinkeIn profile. I don't though, because the last thing I need is more recruiter spam.
Having worked as a Systems Administrator though, you tend to have a positive impact on the lives of many people beyond your own field of employment. At least, if you are doing your job right you do.
When you have been working for a quarter of a century it gets hard to keep track of some of the finer details involved and it would be extremely difficult to keep in touch with some of these people by phone or mail, that's where LinkedIn shines.