Question

how does the use of celebrities help in public service campaign ​

Answer 1

A celebrity has support of many people who follow or admire them.

By being famous and familiar among the people,

celebrities earn a recognisable platform, wherein whatever they do or advertise for, is appreciated and considered by many of their supporters.

Hence, in a social cause or campaign, the use of celebrity helps in popularizing the motive of the campaign and invite many people to it.