Redirects are one the hidden weapons of SEOs. Use it well to ensure your web pages get indexed properly by the search engines.
Redirects can help you in the following ways:
- Create friendlier URLs without the ugly characters such ?, =, &, %, etc.
- Redirect the non www version to www.yoursite.com.
- Move an existing site to a new one.
A good summary of this along with some great 301 redirect tools can be found here.