I call them 'crutch words'. They help to keep the flow of the discussion, while you actually think about what you're going to say. As for everything, overusing them is no good.
Sometimes, non-native speakers just think it sounds cool, or believe it will make him/her appear more fluent. If the conversation is riddled with mistakes, this is the same effect as wearing Church's or Berluti's shoes with a Nike track suit.