Your pool brush attaches to the telescopic pole and is most commonly used to brush algae off of the walls. To quote the Pool Janitor; "Plaster pools like to be brushed..." Brushing your pool will keep dirt from occupying the small pores and starting small organic farms. Steel bristled brushes, called algae brushes, are very effective on, you guessed it, algae. Do not use a steel brush on a vinyl lined pool.
Done regularly, brushing can also reduce the time spent vacuuming. Brush from the shallow end towards the deep in overlapping strokes. Circle the pool towards the main drain, and much of the dirt will be swept up into the filter in this manner.