This is a fairly generic principle but it is not always widely known or remembered. But it is one that often causes support issues..
Here is an example scenario:
You set up a Windows Server 2003 server from the original RTM (a.k.a first release) media.
You don't install IIS.
You update the server with (say) service pack 2.
You later decide you want IIS on there.
You go to Add/Remove Programs and add IIS as a Windows component.
During this step you are prompted to insert the original CD/DVD for Windows Server 2003.
What happens now?
If you are lucky it may work but the IIS part of the server is not be patched up to the SP2 level.
If you are unlucky IIS works partially but suffers some kind of weird failure or error that is hard to diagnose. e.g. the application pool process (w3wp.exe) may not start, for some inexplicable reason.
What is the problem?
You now have a mishmash of DLLs on your system. Some are SP2, some are RTM. That does not work too well.
What should you do?
If you add a new components like this, reapply the service pack.
Will this get any better?
Yes. In Vista and Windows Server 2008, when you install the OS, everything gets put onto the hard drive even if you do not install that particular component. That's why it takes more disk space. But this extra disk space is a small price to pay (and getting smaller daily) for the benefit that when you apply a service pack or a patch, all the DLLs etc get updated even if you do not currently have that component or feature installed.
I like that a lot.