Museums serve no purpose? You don't go to them to take the Model T's around the block do you? You only look at them, to see where cars started, and how they have progressed to now.
Old software may serve you personally no purpose, but for software developers, they can be idea's. I can take a piece of software an examine the layout, structure and many times, the actual code, this is the reason why SCO has released their older UNIX code, for historical reasons, to examine the code. Even the license describes the software as Ancient.
The UNIX code won't do anyone any good to compile and run, but being able to look at the way the code is written can help out many people.
Another reason is just because you personally can't do anything with the older software, does not mean someone else can't find a use for it, such as the many people who still insist on older version of Napster, so they can use the built in media player.