What might be the reason why the bromobenzene substrate is added to the reaction flask in two portions rather than all at once?

Bromobenzene substrate is added in two portions rather than one. The reason for this is because adding it all at once would cause a decrease in phenylmagnesium bromide and increase in the product 1-phenylthanone. The synthesis of 1,1-diphenylethanol would not be able to occur without phenylmagnesium bromide.