Why did a Baroque trio sonata often require four instruments to perform?

Although the term "trio sonata" suggests a three-part structure, it typically required four instruments to perform because of the Baroque practice of basso continuo.1

The genre was written in three musical parts: two high melody parts (usually played by violins) and one bass line. While the bass line was played by a low melody instrument—such as a cello, bassoon, or viola da gamba—it was simultaneously supported by a keyboard instrument, such as a harpsichord. The keyboardist improvised harmonies based on the written bass line, often guided by musical shorthand known as figured bass. Consequently, the "trio" refers to the three written musical parts, while the "four instruments" reflect the necessary combination of two melody instruments, one bass instrument, and one harmony instrument.12