Comment 3 for bug 815552

For one that suffers from the same problem as freehand stroke smoothing (#505180), when you are done it's set in stone and that's that. Furthermore, it can't be applied retroactively. A path effect (sorry about the 'modifier', too much Blender) on the other hand is much more general and could be used, among other things, to interactively reshape existing curves.

It also seems to only useful for relatively minor, parallel adjustments, trying to extend breaks it badly, for example. The ability to extend and join curves as in the second example in the attachment would be the major feature of this path effect, the interactive and after-the-fact parts are just benefits of using the modern path effect infrastructure.