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I would use it whenever I didn't know to whom I was writing. It is best to try and find the name of a responsible person to whom you can address your letter. It's usually not that difficult these days. If it's a cover letter for an application I will admit I often just write, "Hello," and start that way, rather than "To whom it may concern,". Remember for applications we often only have seconds to make an impression and hold/gain interest.
