Media is often put on a physical medium or in a "time slot" that does not perfectly fit, so filler is added to the material of greater relevance or quality to fill a certain time slot or physical medium.
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Television
In the early days of television, most output was live. The hours of broadcast were limited and so, for test purposes, a test card was commonly broadcast at other times. When a breakdown happened during a live broadcast, a standard recording would be used to fill in. On the BBC, a film of a potter's wheel was often used for this purpose, filmed at the Compton Potters' Arts Guild. Similar short films, such as a kitten playing, were also used as interludes or interstitial programs to fill gaps in TV schedules. In the US, these have their roots in the old Saturday afternoon horror movies hosted on independent stations.
Music albums
Albums of music were typically of a set size determined by the physical medium such as the vinyl record (typically 22 minutes per side) or CD (maximum 80 minutes). It was normal, especially in the 1960s, for artists to attempt to "pad out" their material to the standard length by including filler tracks of lesser quality.
Often songs written by the artists or the producer of an album were included as filler and/or released on the b-side of singles to generate more royalties for the songwriter or artist.
Cover versions are often considered to be fillers, though this judgement varies with the amount of creative interpretation and adaption of the original. Similarly, live recordings, demo versions or remixes follow the same argument.
On the subject of music downloads, Courtney Love told the Digital Hollywood conference "If you're afraid of your own filler then I bet you're afraid of Napster", meaning that other artists may be afraid of listeners being able to listen to a full album before buying it.