According to Apple, this patch "improves the security of Quartz Composer and QuickTime for Java." Specifically, it patches a vulnerability that may lead to "information disclosure" when visiting a malicious website (aka phishing site). Complete details on the flaw can be found in Apple's Kbase.
Mike and I were curious about why there's both a PPC and a UB version. Seems redundant since UB means it works on both PPC and Intel. Turns out it's not truly "universal." As Mike noticed first, the System Requirements state that it's Universal for 10.4.8 Server, but 10.48 Client is Intel-only. Hence the standalone PPC-only download (which can be used on client or server.) Would still be nice to streamline this a bit more.
Faithful guinea pigs, please report your experience with this update below!