A person lands on your twitter profile. Based on your profile, and your first few tweets, a person will decide whether to follow or not. Having a pinned tweet at the top of the page gives audience consistency. Everyone that lands on the profile will use the same tweet in their decision about whether or not to follow. This generates a much more focused twitter following.
Suppose you have 40,000 followers. You've grown them, organically over time from regularly tweeting about all kinds of different events that you've attended. Now you want to start a new business, what is the value of these 40,000 followers?
The trouble of course, is that when you start to use your tweet stream to focus on your new business topic, it will be less relevant to a lot of people. Some of them may stop paying attention to your icon as it flashes by their Twitterfeed, others may even unfollow.
The longer term value of those 40,000 followers is approximately the size of the largest interest group, because these are the group of people who are interested in what you have to say now. The others will drop away. However, if all 40,000 share a common interest - and that is the interest that you tweet about - your engagement rate will be exceptional.
Which is why you should use a pinned tweet to define both your topic, and the keywords you want to be known for.
Tools like zenlikefocus.com can take this targeting to the next level. By reaching out to very specific types of account - curated lists, specific job titles, industry verticals - one can generate a twitter following with a high engagement rate.