If you go to Search Console under the Acquisition tab and then click Queries, you’ll be able to see different searches (aka keywords) that users found you through. You may have to set this up, but it’s easy to do and only takes a few minutes; you can see how here. Use this information to evaluate and monitor your keywords to see what’s working and what needs to be changed.
The relevant keywords that you target with your ads will bring the right audience to your website. Showing your ads to people that type relevant keywords will result in higher click-through rate (CTR), lower cost-per-click (CPC) and higher conversion rates for your business. As a result, you will spend less money on advertising and generate a better return on investment.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.