Tracing the story chronologically makes the answer clear. Through the late 1970s and into the 1980s, the Supreme Court began hearing cases brought not by the injured persons themselves but by lawyers, journalists and social workers writing on their behalf, often construed simply as letters to the Court.
Cases about undertrial prisoners left in jail for years, bonded labourers unable to travel to a courtroom, and pavement dwellers facing eviction, none of these litigants could realistically have filed a formal petition in their own name. The Court chose to hear these matters anyway, reasoning that the traditional insistence on a personally aggrieved petitioner would leave the poorest and most vulnerable people without any real access to justice.
That sustained practice, repeated across many cases through the decade, hardened into a recognised doctrine allowing any public-spirited person to invoke the Court's jurisdiction for the enforcement of someone else's fundamental rights. That doctrine is Public Interest Litigation.
Judicial review of administrative action and the expansion of writ jurisdiction under Article 226 are both older or separate ideas about the scope of the Courts' powers, neither one specifically describes this shift in who may bring a case.
\[ \boxed{\text{The evolution of Public Interest Litigation.}} \]