This is a relatively new requirement. Search engines and human directories have started writing alga rhythm's to call out sites with problematic HTML code. Most web designers are dumbfounded by the amount of critical errors found in the pages they have designed. If your site contains more than a few HTML errors you should join NetMechanic . These tags are used by search engines to index your site.
Robots score a sight using a descending value alga rhythm.
Think of it as a thermometer, with each detraction the thermometer shows less value or goes down. So while every site starts at lets say 100 degrees for the sake of simplicity, subtractions are made for a number of criteria. Poorly constructed META tags are the number one reason a robot spidered search engine will exclude your site from inclusion. There are only four mandatory tags that you have to include though its best if you include all relevant tags. 1.Title - Your title should be 50 characters long including spaces and all punctuation.