In the Baroque and Classical periods, conventions dictated how composers would use the instruments. In the Romantic period, composers began to move away from these conventions. By the end of the Romantic period and into the 20th century, instruments were increasingly selected for the qualities of their sounds. Debussy and Ravel were particularly well known for this.
Timbres were often chosen by Romantic composers to create a specific effect. This video shows how Saints-Saëns uses timbre in his Danse Macabre.
It is important to learn the differences between sonority, instrumentation and orchestration.
Instrumentation refers to the instruments or voices used in a composition. It also refers to specific instrumental techniques. Specific instrumental techniques are often used to create different timbres.
The orchestration of a piece of music is how the instruments are arranged together or scored, ie which instruments (or instrumental families) play the melody and which play the accompaniment.
Instrumental techniques are the ways in which an instrument can be played. Here are some examples: