Models that include a cosmological constant can explain recent supernova observations, which imply that the expansion of the universe is accelerating with time.
Certain Grand Unified Theory (GUT) models lead to a cosmological constant during at least part of their description of the early history of the universe.
Introduction of a cosmological constant simplifies theories of the universe and reduces the number of tunable parameters needed to explain its properties.
Introducing a cosmological constant provides an additional method for theorists to fine-tune their models to match several observations at once.
Such models may resolve conflicts between the observed ages of stars in globular clusters and the age derived from the Hubble expansion.