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Try to picture a process that receives trace events generated in your system. You might want to do many things with these trace events, but you might not necessarily want to do all of them at the same time. You probably want to log all the trace events to file. If you are in front of the console, you might want to print them to standard I/O. You might be interested in statistics to determine how often certain errors occur, or if the event requires some action to be taken, you might want to send an SMS or SNMP[18] trap.
[18] SNMP stands for Simple Network Management Protocol. It is a standard used for controlling and monitoring systems over IP-based networks.
At any one time, you will want to execute some, if not all, of these actions, and toggle between them. But if you walk away from your desk, you might want to turn the logging to the console off while maintaining the gathering of statistics and logging to file.