Understanding the balance of terms that might be a little more difficult due to competition, versus those terms that are a little more realistic, will help you maintain a similar balance that the mix of long-tail and head terms allows. Remember, the goal is to end up with a list of keywords that provide some quick wins but also helps you make progress toward bigger, more challenging SEO goals.

And you’ve got to diversify. Here’s a tongue-twister that’s absolutely true: diversity is a key word in the keyword world. You’re not going to stand out if you find yourself using all of the same keywords as your competitors. Not only should you try new keyword search tools and keep track of the results, but you should feel free to experiment based on your own research – who else uses your keywords? And how do you make yourself stand out? By providing great content that truly answers the questions your prospective customers are asking with their keyword searches.
******” Quote from Google: One other specific piece of guidance we’ve offered is that low-quality content on some parts of a website can impact the whole site’s rankings, and thus removing low-quality pages, merging or improving the content of individual shallow pages into more useful pages, or moving low-quality pages to a different domain could eventually help the rankings of your higher-quality content. GOOGLE ******
The SEO starter guide describes much of what your SEO will do for you. Although you don't need to know this guide well yourself if you're hiring a professional to do the work for you, it is useful to be familiar with these techniques, so that you can be aware if an SEO wants to use a technique that is not recommended or, worse, strongly discouraged.