Suppose you like to find out how to prepare Idli and try to search in Google by writing a complete sentence. Now, every one may write various sentences to describe it. Some may write "How to prepare idli?", another may write "How idli should be cooked"? etc. The question is the same but the wordings of each one will be different. The main issue here is idli and its preparation so the keyword is idli and its recipe. Just search by "idli recipe" and a lot of them will be shown. Any subject contains a lot of things and it is impossible to search for anything specific without its mention. Hence keywords are used to make the search process simpler.
"Life is easier when you enjoy what you do"
Even when we go to a physical library and search for some subject we will go and search by the name of the product about which we are searching and that only will give us the complete status of that product as of that date.
As mentioned by Sankalan, if you see books the index will be given on a keyword basis only. If the indexing is done based on the keywords it will be more useful to the person who is searching for some information. The indexing is to do the search fast and at the same time more useful to the user.
With the help of Keywords all search engines like Google, Yahoo Bing etc can reach your Articles. Almost we know that 90 to 95% of the traffic in any blog depends on their keywords. So we can say that search anything in Google then we get the results of keyword specific. It is the main algorithm of search engines
Success occurs when opportunity and preparation meet.
Knowledge is power.