No idea what's different, I couldn't spot anything obvious, so presumably all under the hood fixes. You'll be prompted to update when you start up the existing Store client, so it's not as if you have to go looking for the download.
The update is quite major as a stability milestone though, as it brings all phones from about 2007 to 2010 (inclusive) up to a common Store version and I'd assume that at some point back-end changes in the Store will start to require v1.24(9) or higher on the phone, regardless of actual device model.