Distilled water used in aspirin titration?

I am using a titration method to determine the content of aspirin in different brands. The aspirin is added to 50 cm^3 0.1M NaOH with bromothymol as the indicator. This is meant to be titrated with 0.1M HCl... it was suggested to me that i add distilled water (so that it goes to the 250cm^3 mark) to the solution of aspirin and NaOH... what effect if any does this water have? why is it used?