Keeping things legal helped the Panthers make alliances. Their allies were weak reeds, though. When the USA government made concession to leftist causes, the Panthers lost power. They'd made common cause with leftist groups; when the USA stopped the military draft and promised an end to fighting in Vietnam, those allies mostly stopped protesting, didn't hold out for racial justice.
Finding their sweet spot was tricky; the Panthers didn't always get it right. This was fine-tuned politics, and we might not ever know what some of the background thinking was. Some Panther leaders survived to write autobiographies; but some were assassinated by police. (Though this book concentrates on the politics, that's intertwined with the police violence, provocations by the FBI's COINTELPRO,…) Don't get too attached to any "character" in this book; plenty met violent ends.