What is the added benefit of adding a style to an existing table? (Microsoft word 2007)

The added benefit of adding a style to an existing table is that it will allow to you to change the format of a table and also maintains "professional appearance". This provide consistent "borders and shades, fonts, alignments". You can "change" the style of the table "any time", without affecting the data.

The data will get copied automatically to the newly created table. To insert the table style, click on the table for which style is required, on the "Design tab" select "table style". This provides easy shift to any design, any time without working manually.