What is the importance of FADH2 and NADH?

They are used in the production of ATP, the 'energy currency' of the cell.

FADH2 and NADH have the very important role of being electron carriers for the electron transport chain. Both are used at key points in glycolysis and/or the Kreb's cycle.

FADH2 : Carries electrons in the form of hydrogen atoms (1 proton and 1 electron) with 0 net charge. With 2 hydrogens FAD may use these electrons one at a time or 2 at a time, depending on which ever process it is in.

NADH : Carries it's electrons in the form of hydrides (1 proton and 2 electrons) with -1 net charge. Since these electrons are in the hydride form they both must be used at the same time, there isn't an option to use 1 OR 2 only 2 electrons.