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They exist to allow network television stations to send content to smaller local stations. The shows contain no commercials, just a small gap of blank video to allow the local station to insert their own. The exception for this is for "barter" syndicated programming, where only the commercials required to be shown are included, with black space provided to mark local commercial time.
Some universities, colleges and larger schools have the equipment available to receive these signals, as do a few individuals. Some people record the preaired show onto their computers and upload it to the internet to allow people around the world early access. However, this is a copyright violation. Apparently due to this many networks have switched to a digital feed which requires more expensive equipment and can also be encrypted if needed. This has reduced the availability of preair episodes appearing on the internet.
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