why silver nitrate needs to be acidified to test Cl ions??

Because silver carbonate and silver sulphate,etc, are also insoluble in water, but they would react with acid to dissolve. So you need to acidified it to ensure the precipitate formed are silver chloride but not other silver compounds.

Because any base appears in the solution can react with Ag+ to form greyish colour ppt which has similar colour of AgCl. We will then have a fault test result.