Most advertisers' first stage in planning a PPC campaign is to immediately go to any one of the numerous keyword suggestion and search volume tools to determine which words are generating the most traffic. The traditional thought is the higher the search volume for a word the more value that word has, and therefore the higher the bid.
The simple fact is that there is a major difference between keywords that work and keywords with high search volume. The secret to success lies in knowing your target customer and being able to speak their language. It does not matter if a seemingly relevant keyword has tremendous search volume, if it is not representative of the language the target market is speaking.