If you are working with a process that has many of your DLLs loaded in it, and your DLLs tend to be loaded and unloaded dynamically, then you can sometimes save yourself a lot of trouble when debugging a problem by making sure that each of your DLLs has a unique base address.
That way, if you have a bug where one of your DLLs is called after unloaded, you can easily figure out which DLL the call was supposed to go to and which function it should have gone to by loading the DLL using the dump file loading support (start WinDbg as if you were going to debug a dump file, but select the .DLL in question instead of a .dmp file) and unassembling the address that was referenced.
On Windows Server 2003 and later, NTDLL maintains a list of the last few unloaded modules and their base addresses in user mode (accessible using “lm” in WinDbg) which can make debugging this kind of problem a bit more manageable even if you don’t rebase your DLLs, but rebasing is easy, improves loading performance (and especially scalability under Terminal Server), so I would highly recommend going the rebasing route anyway.
If you weren’t on Windows Server 2003 and didn’t rebase your DLL, then chances are the loader relocated it at load time to some not-predictable location, which makes finding the actual DLL being called and which function / global / etc in the DLL was being referenced when the crash occured much more difficult than if the DLL always loads at its preferred base address and you can simply look up the address in the DLL directly.