Fuel Cell foam is generally regarded as being used to prevent fuel slosh (similar to baffles).
While it does serve this purpose, its also used to stop a flame front from advancing, helping to eliminate the possibility of an explosion.
Should the tank receive a sudden impact, the foam inside the fuel cell helps dissipate and absorb a great amount of energy from the impact, reducing the disk of fuel cell rupture.
The tank should not be completely filled with foam, as it expands upon contact with the fuel. The fuel cell should have no more than 75% of the area filled with foam.
There is no set life of the foam, and can last up to 10+ years without needing to be replaced. The inline fuel filters should be checked often.