Another use I have found recently is using a log file to figure out what is going on in a child, during development. I had some legacy code and this child was sending email, it was being ported to windows.. well it was very useful.

Sometimes I use shell output redirection so I get stderr and stdout both going to the same file, to log what happened with a system call. I don't want to post the wrong info but it is something like &2>1, see man bash.