Pardner is not a different word with a different job — it is partner said in a cowboy, old-Western way, and people write that sound as pardner for fun. You use it humorously to someone you know well: Howdy, pardner, or Come on, pardner, often with a pretend hat-tip or a film voice. It is playful, informal, and American-flavoured. You would not put it in a serious email, a school essay, or a job letter. Older Western films made the joke famous; younger speakers still use it as a wink, not as their real word for “friend.”