Siliconslave's how to make espresso thread

[Kainz]

I think there are other factors too. As my beans age the ground weight for a given time changes.

Pretty much this. As beans age you have to grind finer as your initial dial in settings slightly change. Weighing and timing enables repeated consistency.
For example I lob my beans into a small airtight tin. As I work my way through the beans, by the time I've reached the bottom I'd have made a small tweak to the grinder so I can keep to my 18.5g in, and 36-37g out in 30-32secs.

Raymond also mentions flow in his post. Some of these scales will give you a flow rate. Thanks to that feature I can tell in seconds if my shot is flowing too fast or slow and immediately stop it.