Identifying trends in language, or more specifically, identifying the words and terms people search for on the internet can be extremely helpful in discovering which words and phrases you should, or shouldn't use on your website to attract new visitors.
Lets say you were starting a business and were going to sell "widgets". You are getting ready to put together your marketing material and source vendors for product, but you aren't sure if people were interested more in "red widgets" or blue widget".
Doing a web search and simply observing the number of listings found on a search engine for either really wouldn't give you any insight into buying trends. But, if you could know which term more people search for, and whether the trend was increasing or decreasing, you would have a much better idea.