Why do forums use this url format?

From a purely pragmatic point of view. It doesn't matter which approach you use.

The problem being solved by both methods is the ability target a page and pass in parameters.

Arguably using www.domain.com/post/1234/Latest is a more readable URL.

If you look into web frameworks such as MVC you see why/how these URLs are built up in the way that they are.

They implement a concept called routing. The following is an example:

public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.MapRoute(
        "Default",                                              // Route name
        "{controller}/{action}/{id}",                           // URL with parameters
        new { controller = "Home", action = "Index", id = "" }  // Parameter defaults
    );

}

This example shows how the URL is built. The 1st part is the controller, the 2nd part is the action to apply and the 3rd is the id.

So for the following URL:

https://www.mydomain.com/customer/get/1234

You can see that you're accessing the customer controller and you're executing the get action on record 1234.