Why does Punjabi use tones?

In the context of Indo-Aryan languages, the presence of tones in Punjabi is a distinctive phonological feature that resulted from the historical development of voiced aspirated stops.21

Specifically, the tonal system emerged through the following shifts:

• Initial Position: When a word began with a voiced aspirated stop in earlier Indo-Aryan, it evolved in Punjabi into a voiceless stop, accompanied by a low tone on the following vowel. For example, the Hindi word ghora ("horse") corresponds to the Punjabi kòra.21
• Non-initial Position: When a voiced aspirate occurred in a non-initial position, the consonant became unaspirated, and the preceding vowel developed a high tone. For instance, the Hindi dudh ("milk") appears in Punjabi as dúd.21

This process of realizing historical aspiration as pitch contrasts sets Punjabi apart from other members of the northwestern group of Indo-Aryan languages.1