The most popular query or two will usually be your brand, no surprise.
But look further down and you'll start to see phrases you'll be surprised by. These "Oh, I'm getting traffic from that?" keywords are easy opportunities to score more traffic from Google. Especially the ones with a lot of impressions.
When you see a keyword like that, just click the link icon next to it to see which page on your site is ranking for it, and then check whether that page actually has that exact phrase anywhere in the copy.
If it doesn't, write the phrase into the page copy somewhere. Now you're almost guaranteed to get more traffic from it going forward.