.htacess file is used to solve the canonicalization issue of a website. It may happen that the home page of the site may take several urls like
1. http://www.abc[dot]com (with www)
2. http://www.abc[dot]com/index.html (Index page)
3. abc.com (without www)
The search engines might treat these URL as different and may divide the link juice gained by having various backlinks made with any of these 3 URLs. The link juice gets divided between these three URLs equally. .htacess file is created to have a single URL of the home page so that the link juice is passed onto single URL.