Why is the cosmological constant important in anthropic explanations of the multiverse?

In the context of the multiverse, the cosmological constant is significant because it presents a major "fine-tuning" problem that anthropic reasoning attempts to resolve.1

Theoretical predictions from quantum field theory suggest a value for the vacuum energy density that is vastly larger than the observed value—a discrepancy of approximately 120 orders of magnitude. In a multiverse paradigm, which posits the existence of a vast number of disconnected universes with varying physical constants, this unexpectedly low observed value is interpreted through the anthropic principle.1

The argument suggests that among a multitude of possible universes, only those with a cosmological constant small enough to allow for the formation of galaxies, stars, and ultimately life, can be observed by living beings. Because a larger constant would cause the universe to expand too rapidly for large-scale structures to form, our existence acts as a selection effect, explaining why we find ourselves in a universe with such a finely tuned, low value.1