Type 1a supernovae have distinguished themselves in astronomy as "standard candles." That is, their peak brightness is predictable based on observations of the light curve, or how the brightness changes with time. These white dwarfs probably detonate when they collect too much mass, getting closer and closer to the Chandrasekhar limit where they can't support themselves anymore.
PHOTOS: Top 10 Spitzer Nebula Stunners
This predictability in brightness means that they can be used to accurately measure the distance to very distant galaxies. They are an important rung in the "cosmological distance ladder," or the way we measure distances in astronomy, and were famously used in the discovery that our Universe's expansion was accelerating due to dark energy.
Are all Type 1a supernovae really alike, though? There is evidence that the amount of heavy elements in the progenitor white dwarf can affect the brightness, introducing an uncertainty in distance calculations. These uncertainties haven't been enough to overthrow indirect observations of dark energy, especially in light of other lines of evidence, but astronomers are always after more accuracy.