No, it's there in case legal action ever needs to happen to enforce licence terms against an infringer. Say for example a Great Big Musical Instrument Company decides to use EigenD to power a keyboard (unlikely I know, but this is just an example) and does not release the code under the GPLv3 (or they Tivioise the device, which is prohibited by v3), and we or perhaps the FSF need to sue them, we may need an affidavit from you (if you contributed) for court. Or if you became a big contributor, you might be asked (at our expense, as it says) to testify that you wrote he code you contributed.
All this is pretty unlikely and made even less likely by the existence of the clause, but it is very common in copyright assignment clauses. If you work as a software developer you probably have one in your employment contract for your employer just like that - copyright assignment without the assignee pledging to assist in the legal defense of that copyright is a bit useless in reality.
Hope that makes it clearer. We derived this from the Ubuntu one for several reasons, firstly it saved me a legal bill (always nice), secondly because a lot of open source developers have already committed to it and lastly because its a well drafted and considered template.
John