why is excess co2 used when reacting phenylmagnesium bromide grignard with carbon dioxide?

You have a Grignard reagent, that is very sensitive to any traces of moisture. The best way to do this reaction to make a carboxylic acid is to crush a large quantity of solid CO2, and simply pour the reaction mixture directly over the crushed CO2.  You have overwhelmed any moisture present with huge amounts of CO2.  And you won't get any exotherm either.  And the CO2 simply sublimes away, and you then work up the reaction.

You do not add CO2 to the reaction mixture.  You quench the reaction mixture by pouring it over crushed CO2.