What is the use of legacy URL routing mechanism in Joomla 1.5

I came across this option of enabling it in the Legacy plugin of Joomla. Now I know that Legacy Plugin is used to make older templates of Joomla compatible with the new one. But what is Legacy URL Routing Mecha开发者_运维问答nism. Please answer it in easily understood language - I am a noob.

The Legacy plugin is actually used for most 1.0 extensions, not just templates. This was supposed to be a temporary solution for running 1.0 extensions on 1.5 sites until developers had a chance to rewrite the extensions for the newer MVC architecture. J!1.0 used a different routing system for search engine friendly URLs which is not compatible with the 1.5 routing system. In order for extensions running in Legacy mode to have SEF URLs you would need to enable this option in the plugin.

That being said, I wouldn't use an extension that requires legacy mode at all. Not only is there a small performance hit, but code that old has potential for security issues. It's been 3 years, it should have been rewritten by now. Not to mention there is no Legacy mode in 1.6 so there are no upgrade options for that extension.