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Description and Title Tag are very crucial piece of information not only for Search Engine Optimization but also from usability perspective.
The keywords appearing in the Web Page Title & URL are highly weighted by the search engine and you must make sure that target keywords are appearing in Web Page Title or URL.
While search engine may use this information for ranking in search results, for search users the information appearing in the Title & Description is the decision making factor to select from multiple search results. If description tag is missing then Search Engine automatically build a page description but that may not necessarily reflect the best summary of page contents.
Please note that there are limitations of number of maximum characters we should use for Title & Description tags that Search Engine may use while displaying the search results and excess data is ignored. The optimal length for Web Page Title is 70 characters and for Description 140 characters.