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The gas does absolutely noting as far as "cleaning". They are inert gases used to sheild from impurities during the cooling of the weld. It really doesn't even do any cooling (put your hand oven the nozzle and tell me if you think the little that comes out would have any cooling effect!). Running argon over a surface before tig welding is a waste of gas. I find it hard to believe that someone who knows how to tig would even think this - especially a Canadian ;-) !!

Different mig gases will have an effect on a mig weld.

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Going over the mat'l first with the torch etches the metal. Do me a favour, the next time you are TIG welding contaminate your electrode by touching the metal while welding. Now go regrind your electrode and go over the dirty mess left by the contaminated weld. Where does all the nastiness go? I'm no scientist but maybe the impurities on the surface (ie:oils, oxides)are changed to gases and are then shielded from the area to be welded. All I know is that it works.