New Post: Form Enhancements are synchronous - why?

Some Form Enhancement functions provided make synchronous calls to the Sharepoint Web Services. Why is this?

For example, SPFilterDropdown calls GetList and GetListItems with async: false.
Comment in the code reads:

// Force sync so that we have the right values for the child column onchange trigger
async: false,

I'm not exactly sure why it's important that it's synchronous here. Could anyone explain it to me?