The test offers an opportunity for charities to help the public understand them, she said in an interview with Third Sector.
"Charities are an essential part of civil society and communities, for which society rewards them with tax breaks and trust," she said. "Charities must earn that status, and I think it's important the wider public sees what the benefit is. We need to close the gap, in a sense, between what the public thinks charities are and what they do."
This is where the public benefit test comes in. She said: "I think trustees will look back in five years' time and think: 'This is what brought us back to basics, to our core aims, and made us start thinking seriously about how we explain our work to the public.'"
Leather was keen to reassure charities in the wake of the huge response to the consultation on the draft public benefit guidance. "We really underestimated the potential for anxiety and concern that the removal of presumption of public benefit for some causes would have," she said. "We have done a huge amount of work to reassure people and to clarify our message."
No more than a handful of charities will lose their status, Leather said. She was also adamant that the commission was not out to "secularise the register".
Nearly 60 per cent of the responses the commission received were from religious charities, some of them outraged.
Leather was keen to emphasise the work the commission would be doing to help charities meet the public benefit requirements and report on their credentials.
"We want to build best practice and educate charities; it's that process that will drive public benefit," she said.
"I don't think the public benefit is that complex: we have produced guidance that reads well and will be an enormous help for trustees. We're not at the end of the road yet, but we will work hard with charities to ensure they can deliver."
Quotes from responses to the consultation
'It's important that the commission has stated that the relief of public funds is of benefit to the public'
'It should be made clear in the guidance that evangelism, proselytism and the propagation of religious belief are benefits, not disbenefits'
'Encouragement of irrational thinking, 'blind faith' and action on the basis of beliefs so held could be argued to fall within one of the disbenefit categories'
'If public benefit is measured only by activities that relate to the charity's articles, would we be less inclined to offer our facilities and expertise at a charitable rate?'