That’s the point. The point of the clock is to put a frame on a span of nearly unimaginable time and live life now in a way that will be significant in 10,000 years - it is a challenge to do big, great, meaningful things with long term impact.
The clock itself as a mechanical object is an expensive monument and nothing more, it likely won’t last the full 10,000 years of course...
That does not make much sense to me. You can't both admit we have no idea what the world will be like in ten thousands years, and at the same time claim you can do something that will be significant in such time frame. You don't know what will be significant then.
There are one more point. This clock is a signal to a future, and this clock will have some meaning for our ancestors. We do not know what meaning it will be exactly, but we can be sure, that there will be some.
It is like the Stonehange. We don't know why ancient people bothered to build Stonehange, but we inspired to think about it.
This style of thinking sometimes has its benefits, like gathering together a community to build a mechanical clock, create a manual for restarting a civilization[1], or write a non-fiction book about "earth without people"[2].
It's just an interesting intellectual, cultural endeavor.
[1]: http://blog.longnow.org/category/manual-for-civilization/