title_body: Why do we need to fold the border of filters?
upvoted_answer: I did a little bit of research at the site of inventor of the coffee filter. It's Melitta from Germany. In the german FAQ I found the following:
  
  Warum falte ich Filterpapier vor dem Einlegen um?
  Damit die Kaffeezubereitung optimal funktioniert, muss das
  Filterpapier genau in die Form des Filters gebracht werden. Dies
  geschieht, indem Sie die beiden geprägten Ränder umfalten. Das
  Filterpapier schmiegt sich dadurch an den Rand des Filters an und
  sitzt tief im Filtereinsatz. Durch diese gute Passform kann sich das
  Filterpapier während des Brühvorgangs nicht einklappen
  
  In English:
  
  Why I fold the coffee filter?
  For a good result the coffee filter must be in exactly the form of the
  drippter. This is done by folding over the two embossed edges. The
  coffee filter is nestled by the edge of the dripper and sits deep in
  it. Through these good fit, the coffee filter can not collapse during
  the brewing process.
  
  Additional fact: The coffee filter was not invented by Melitta the company, but by Melitta Bentz.
  
  Sorry for that bad translation. My English is really not the best. Please improve me!