The object element allows you to nest alternative content inside of it, which will be displayed if Flash is not installed or supported. This content will also be picked up by search engines, making it a great tool for creating search-engine-friendly content.
Summarized, you should use alternative content for the following:
- When you like to create content that is accessible for people who browse the Web without plugins
- When you like to create search-engine-friendly content
- To tell visitors that they can have a richer user experience by downloading the Flash plugin