What is the use of keyword planner?

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Research keywords: Need help finding keywords to add to a new campaign? Or, maybe you want to find additional keywords to add to an existing campaign. You can search for keyword and ad groups ideas based on terms that are relevant to your product or service, your landing page, or different product categories.
Get historical statistics and traffic forecasts: Use statistics like search volume to help you decide which keywords to use for a new or existing campaign. Get forecasts, like predicted clicks and estimated conversions, to get an idea of how a list of keywords might perform for a given bid and budget. These forecasts can also help guide your decision on which bids and budgets to set.