Google has created a plethora of awesome research tools that can be used for website development. Their keyword tool can be found here
I personally use the keywords tool quite a bit for work. The primary purpose of using this secondary research tool is for search engine optimization. When writing content, whether it be for a websites direct content or for a websites blog I use the keyword tool to aid in the creation of content that will make the front pages of search engines such as Google.
There are several important things to know about the keyword tool:
When searching for a keyword that has high competition it is rarely likely that the websites I work on (small businesses) will be able to rank for the keyword. For instance the local monthly searches for the term "holistic dentist" is 18,100 searches per month. The tool allows me to look at related terms that have lower competition but are more descriptive and will thus be more relavent to the person searching. "holistic dentist mn" has a much lower search result of 110 monthly searches but would be much more applicable to the people looking for a holistic dentist in mn and would be a much easier term for me to rank.
In addition to checking competition of certain key words it also gives you ideas for related key words that can then be added into content in hopes to rank for those keywords as well. I am no expert at this point but from the helpful material that Google provides I have been able to rank in the top listings in Google for several different keywords. If you are interested in checking it out you can search "holistic dentist eagan" or related terms to find one of my websites cedarcliffdental.com