In between Service Packs a lot of 'Hotfix' updates are developed and made available. A number of them get made available via Windows Update as Critical updates because they relate to security issues, but most don't.
Instead, the 'Hotfixes' which relate to specific problems get mentioned or linked in Knowledgebase articles. Quite a few are only made available following a specific request for them to Microsoft Product Support Services. "Only apply it to systems that are experiencing this specific problem" you are told, and "..if you are not severely affected by this problem, we recommend that you wait for the next Windows XP service pack that contains this hotfix."
That's right! They'll be included in the next Service Pack anyway, making the download larger whether you need them or not!
Thanks to website MajorGeeks, you can download a whole bunch of the Hotfixes in a handy zip file right now, forestalling some of the later pain. There are about 120 of the updates included in the archive, and you can download it from here.
Unfortunately there's no installer included. You'll need to install them individually.
What you fail to mention is that a service pack will not just integrate hotfixes, it will also contain thoroughly tested versions of them.
So if a hotfix fixes the problem but introduces a smaller problem elsewhere which isn't critical, the service pack will address that problem by replacing the hotfix by something else (or an improved version of itself) that does not have that flaw.
The SP will also be available for a one-off installation to systems that were installed after it becomes available.
If it would not include those hotfixes people will have to install them all separately, which many will not do or fail to do properly causing problems further down the line.
There's many reasons to make a service pack all-inclusive, going even so far as to have it include all previous service packs.
Knowing the history of Windowsupdate functionality, those installing the SP through that will get only those things they really need and not things they already installed earlier and didn't change since.