Final answer:
Greek legends and Roman history were commonly used as the basis for baroque operas because of their widespread popularity, universal themes, and alignment with the grandeur and spectacle of the Baroque style.
Step-by-step explanation:
Baroque operas often used Greek legends and Roman history as the basis for their stories due to several reasons. Firstly, Greek and Roman mythology and history were widely known and appreciated during the Baroque period and provided rich sources of dramatic content. The use of these stories also allowed composers and librettists to explore universal themes and emotions that were relatable to audiences of that time. Additionally, the grandeur and spectacle associated with Greek and Roman culture aligned with the artistic and theatrical style of the Baroque period, making these stories a natural fit for operas.