Why is PIL called the sole instrument of social revolution?

Public Interest Litigation is called the sole instrument of social revolution for the following reasons:

• It tends to bring justice within the reach of the poor masses.
• It leads to speedy justice.
• It has changed the plight of thousands of under trials.
• Anybody can seek justice on behalf of those who are illiterate, poor, and downtrodden. 

Further Reading :

• Public Interest Litigation (PIL) in India
• Components of Redressal Mechanism