Many people and SEO companies make the mistake in targeting the key phrases with the largest search volumes only without looking at conversions.  If you get 10,000 people to your site through 3 keywords but only convert 3 people through those keywords then that is not a very good conversion rate.  It is important to look for the keyword phrases that are going to convert visitors into sales.

Using key phrases that may drive high convertible visitors to your site each month is likely to be much better than targeting a highly competitive broad key phrase that is only vaguely related to your business, even if it does attract 10,000 searches each month.