When copying the standard output from within a terminal I often push it to a file using the "command > file" syntax. The trouble is that I then end up with a file that I have to open in order to get the output, and I often forget to delete the file once finished. This is especially annoying when I just want to paste some debug output into a help topic or similar. The solution to this is to use xclip to store the output in the xclipboard instead of a file. This is essentially the function of xclip, it allows access to a clipboard that you can store anything in.
A good example of this in use is when pasting the output of php -i into the amazing xdebug tailored instillation instructions. When you print this to the terminal you get pages of output. To push this into xclip instead just pipe it like this.