The Fibonacci sequence is just one of several that are used in project planning poker.
It is difficult to accurately estimate large units of work and it is easy to get bogged down in hours vs days discussions if your numbers are too "realistic".
I like the explanation at http://www.agilelearninglabs.com/2009/06/story-sizing-a-better-start-than-planning-poker/, namely the Fibonacci series represents a set of numbers that we can intuitively distinguish between them as different magnitudes.
