Or you can literally just manipulate your SVG through the DOM in an external JS script... I still have no idea what the original motivation behind scripts in SVGs was.
While SVG is a web technology, for the longest time you had to install SVG support as a browser plug-in. I remember installing Adobe SVG viewer around 2000. It was used for interactive visualizations.
I'm don't remember precisely but I don't think you could script it from the DOM, I don't see how that could work if it's a plugin.
I imagine it may have been attractive to those who liked Flash.