Can someone explain the 'two bucket' method?

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As a total noob to detailing etc. I've been reading up a fair bit before attempting anything but have noticed a lot of people referring to the 'two bucket' method for washing their cars prior to claying / polishing / waxing.

Now I always use one of those microfibre mits to wash my car with Mer car shampoo/wax but just use one bucket which I only use half the water of (so as not to pick out all the muck that sinks to the bottom).

So can someone explain what the deal is with the two bucket method and also recommend some decent products to use as now I have a black car I'd like it to be as shiny as possible!