Note: as a progressive optimization, the very same solution can also be extended to full pages, ie not just "body" parts.
In that case, the main benefit over a "regular" load is that javascript files that you use don't get to be evaluated over and over, ie you load them once for all.
In that case, the main benefit over a "regular" load is that javascript files that you use don't get to be evaluated over and over, ie you load them once for all.
On my machine, this alone saves 300ms per page.