Because AdEspresso’s topic is Facebook advertising–which is a popular topic–the initial keyword list was pretty gigantic. To narrow it down, they went through and highlighted in green the keywords that they thought could easily turn into blog post topics. They also highlighted in yellow the keywords that they thought would be good for backlink anchor text.
A blog highlights your technical ability and showcases your ability to write blog posts. Your blog can be about different topics than those you write about for your clients. In fact, it should be on a topic that interests you. Visitors will see that you can not only write, but you can also build an online community. A good blog has the potential to earn you many referrals for more clients.