Each style of beer has it's own mineral content. Burton water is great for 'English' beer but if you are brewing a Guinness Clone you want Dublin water.
Big breweries base their beers on their water. I'm trying to copy them so I'll have to adjust the water that comes from my tap.
Sure, but most big breweries will still adjust their water chemistry to suit the beer (unless it's ideal I guess - do Marston's fiddle with theirs?); e.g. Fuller's adjust the mineral content of their water.
I thought most breweries Burtonised their water, at least for their lighter beers?