You mean 212.555.1212 ?
Using decimals in phone numbers is something I started to see about 15 years ago. Early adopters seemed to be mostly design-related companies; I first saw numbers formatted with decimals on the letterhead of architecture and graphic design firms. From there, it spread.
Why use them as a standard? I can’t think of a good reason, except that a parenthesis for the area code seems to imply that dialing it is optional, when in fact it’s required in areas with ten digit dialing. The use of decimal places instead of dashes? It looks clean and stylish, in some way.