Why? So that you in the future you don't go insane trying to find an easy to prevent bug.
There are many situations where Hibernate (and yourself) will compare instances of the ID class to see if they are both encapsulating the same primary key. If you don't override both the equals(...) and hashCode() methods you could possibly get a false positive and a false negative. Wrost of all it would probably be very hard to repeat any such problem, thus the bug would be a monster to find. In light of Murphy's Law the bug will most likely show up during a demo with a customer or upper management.