Keeping people up-to-date... indicating the correlation between the previous PR's and the predicted/promised results. It doesn't have to necessarily entice new buyers, it can serve to keep the existing shareholders. Losing one existing shareholder can send the price down by a fair margin, especially larger positions, and short-positions benefit as a result.
PR's show that a company cares about it's stock, pps, and the value of its investors. It also indicates that things are happening. I much prefer updates, even smaller ones, over nothing, followed by more nothing.