External templates allow you to reduce the code in your main HTML page, and reuse it in multiple places.

Selecting a CSS template is a bit like selecting an outfit do it right and you feel good about yourself, you look great people are impressed. Choose badly and your site will either be inflexible, ...

Another option is to generate fresh HTML code for your blog using the Template Designer, and then not edit the HTML further, preserving the original code created by the computer generator.