- see ways to reach your target audience based on your keywords, website, or interest categories
- see which image and text ads are performing best on the specific websites you’re targeting
- see how other advertisers perform on websites where you want your ad to appear
- compare how your current Display Network campaign could perform on websites you’d like to target
Explanation: Display Planner shows you the potential reach and cost of targeting ideas to help you plan your Display Network campaigns. The forecasts can help you understand how your ad groups could perform and historical costs can guide you to the ideas that are within your budget.
Read more here: https://support.google.com/adwords/answer/3056115?hl=en