Keywords has an important role in providing visitors to your site. If a word has been trendy in search engine it means that people are looking for topics that related to it. Suppose thing that 'oscar' is trendy keyword in search engine. If you want to turn the visitors with oscar to your site then your site require a keyword which is related to oscar. So like this keywords has an important role in blogging. If your posts tag is a trend keyword and if your site has a good page rank then when a person search for that trend keyword your site will be visible in the search engine.
In the above eg i mentioned about the keyword oscar if your post have oscar as tag then you will get thousands of visitors in a day itself. If your site purpose is for blogging then think that if you are using advertisement programs like google adsense,chitika etc. And they offer a epcm of 20$, if your site get 10,000 visitors a day with the use of keyword and you got 5000 impressions and out of them if you get 3000 clicks. Then count 3000/1000 =3*20=60$ easily in just 1 day. Remember that here i counted only the amount that you will get from clicks in addition to that you will earn for impressions also. That is why the keyword has a big role in providing both revenue and visitors to your site. Use trend keywords in your site to get more visitors. All the best.