The primary function of a data lake in big data architecture is to store vast amounts of structured and unstructured data in its native format until it's needed for analysis. This allows organizations to retain large datasets without the constraints of traditional databases. Data lakes facilitate advanced analytics, machine learning, and real-time reporting. By consolidating diverse data types and sources, they enable more flexible querying and deeper insights, empowering businesses to make data-driven decisions. The scalability and cost-effectiveness of data lakes are increasingly vital in managing modern data workloads.