A good blog design is one of the important aspects that bind your visitors to your blog. Users prefer to surf a good design blog for a longer time than one with dull appearence or too reflective design. It can automatically create interest in your visitors to read your blog post. Visitors are the one which make your blog run so its neccessary that you keep them happy when they are at your place :).
You can design your own template too, and ofcourse you will be admired for that. I tried to design my own but it went in vain lol :P. Still im working on it so that one day i may come up with one.

Ive come accross many beautiful templates while surfing, wish i could have all of them installed. There are lots of beautiful templates for wordpress blogs available at their official site Wordpress themes
For blogger, there are typical themes available at their official site, so most of the bloggers prefer using external templates. You get thousands of results when you search for blogger templates. Im using theme designed by Jacky Supit a cool theme designer !

I have heared that some themes give problem relating SEO, that they conflict search engines due to stuff like css. I dont know much about this, but will write about this if i find anything.