I personally think it is not necessary. Some people however think it is to let their friends know where they are, but on the other hand it also tells the world that you are not at home, which can lead to burglary, or tracking down etc.
I always need my privacy and I do not want people or the world to know what I am doing or where I am. I use google's latitude, where I have given access for only my best friend/s to see where I have checked in, also to mention, the check-in is not automatic. I have set it to "Remind" so I check-in only if I wish to, or if my friend requests me to check in !