The preservation of the legacy of Jesus, Gandhi, Hasan Al-banna or Regina Jonas, owes more to the needs of their followers than to historical accuracy. Why is this significant for the scholarly study of religion?
Keep in mind that charismatic founders of movements tend to provoke stories (in their adherents) of fantastic deeds and often, of supernatural events which can lead to the confusion of history with religion. In time, those stories become indistinguishable from actual history, as in the case of Jesus. Scholars cannot confirm (and can reasonably deny) many of the stories about him that adherents take as actual history, such as his turning water into wine at the wedding at Cana (a story that most likely evolved from other mythologies, in this case Bacchus, the Greek god of wine).
Christians, however, view this story as historical. Such a view is understandable, as they worship Jesus as God, and thus require that he perform supernatural--scientifically inexplicable--deeds. Understanding this gives us the ability to study how and why Christian beliefs have evolved since its founding.
It's interesting that the question mixes Jesus--a 2000+ year old figure--with Ghandi, Hasan Al-banna, and Regina Jonas, all 20th century movement founders, none of whom are worshipped as gods. All they have in common are that they are founders of important movements: Indian freedom (from British rule), the Muslim Brotherhood, and recognition and appreciation of female rabbis (not really a named movement, but one nonetheless). Each, however, represents an important figure in different religions: Christianity, Hinduism, Islam, and Judaism. In each case, the religious figure was linked to a political movement, each of which took on a life of its own. Religious scholars, focusing on the secular, can study the evolution of various movements outside of assumptions of divine inspiration.
The scholarly study of religion is, above all, secular in nature, meaning it studies religions from outside of any religious belief or viewpoint (as opposed to theology, which studies the nature of the supernatural).
The relatively recent acknowledgement (circa 19th century) that the stories of religious founders cannot be assumed to be historical and more reasonably can be attributed to the needs of their adherents is crucial to the scholarly study of religion in that it opens the field to questioning (and discovering) cross-cultural influences and, while not discarding or completely discounting the original stories of the founders, viewing them with a critical eye informed by various scholarly fields, such as philosophy, history, anthropology, archaeology, sociology, anthropology, world literature, and so on.
Once the doors are thrown open to examining a legacy in light of the needs of its adherents, scholars can better understand how the original stories and legacies evolve.