Why does Vladek build a bunker? Does this bunker help the family survive?

In Srodula, the Germans begin to round up Jews at random. To protect himself and his family, Vladek builds a shelter under a coal bin, in which they hide during Nazi searches. Soon, though, they are moved to a different house. Again, Vladek builds a shelter, this time in the attic and accessible only through a chandelier in the ceiling. The bunkers do help the family survive.