Many people confuse services and daemons. Services listen on ports. Daemons are a kind of process. Services can be daemons. Daemons don't need to be services.
http://www.steve.org.uk/R
The above URL details the steps a program should take to become a daemon.
Reasons you're doing those things:
- disassociate from the parent process
- disassociate from the controlling terminal
- chdir to / to disassociate from the directory the process was started in
- umask 0 to ignore whatever umask you may have inherited
- close your filedescriptors and reopen specific ones to your liking
How to Daemonize in Linux provides code examples in C.
So... why do this?
There are a couple of good reasons. A daemon can be a service as I mentioned above. Daemonizing a service is a great idea, so it can stay running as long as is desired.
Another good reason for making a program a daemon is that it'll keep running even when you logout. You can disassociate functionality from whether you're logged in or not. Once you run it, it'll stay running until it's explicitly killed, or a bug causes it to crash.
Monitoring a system is a good reason to use a daemon. Cron can run processes every minute - but if you need a tighter granularity than that, cron can't help. A daemon can. With a daemon, you can setup whatever timing you want in your "main loop".
You might watch for files to exist, or not exist, or drives to be mounted or unmounted, or any number of other things, using inotify or other means of checking what's going on.
Daemons can be pretty darn useful!
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