What is the purpose of one-shot manga? One-shot manga usually only consist of one chapter, so what is their purpose? Is it mainly for promotion, filler or contest? And do they have their own media (like a magazine that only consists of one-shot or something)?
Manga magazines usually have many series running concurrently with
  approximately 20–40 pages allocated to each series per issue. Other
  magazines such as the anime fandom magazine Newtype featured single
  chapters within their monthly periodicals. Other magazines like
  Nakayoshi feature many stories written by many different artists;
  these magazines, or "anthology magazines", as they are also known
  (colloquially "phone books"), are usually printed on low-quality
  newsprint and can be anywhere from 200 to more than 850 pages thick.
  Manga magazines also contain one-shot comics and various four-panel
  yonkoma (equivalent to comic strips). Manga series can run for many
  years if they are successful. Manga artists sometimes start out with a
  few "one-shot" manga projects just to try to get their name out. If
  these are successful and receive good reviews, they are continued.
  Magazines often have a short life.[50]


Source




  One-shot manga tells its entire story in 15-60 pages, usually written
  for contests, and sometimes later developed into a full-length manga
  series (much like a television pilot). Many popular manga series began
  as one-shot stories, including Dragon Ball, Fist of the North Star,
  Naruto, Bleach, One Piece, Berserk, Kinnikuman and Death Note, among
  others. Some noted manga authors, such as Akira Toriyama and Rumiko
  Takahashi, have worked on numerous one-shot stories in addition to
  their serialized works. Rising Stars of Manga was an annual
  competition for original English-language one-shot manga, many of
  which have gone on to become full-length manga series.


Source

Note: Emphasis mine.