Well! That is a simple question. Because of your site Ranking. You need to optimize your site, work on your site SEO, make your site user friendly and make it as simple as possible. All this tie to you using a well layout and optimized template.
You can’t get all this with a blogger default template except you have a very good HTML experience and web design. If not , you need to download already made template.
Some of these sites are given below :
Are we missing anything? leave comment below. suggest your best blogger template site.