Living from 1805-1879, Garrison was a well-known abolitionist and supporter of women's suffrage. The Boston area was, for quite some time, his home, and he is buried in Jamaica Plain.
In his lifetime, he was the founder of the newspaper The Liberator, where he made his views quite plain to see. It was here, in his words, that he stood for social justice. Moreover, he is one of history's few individuals who possessed the ability to admit wrongdoing. By the end of 1830, he had publicly denounced his previous support for colonization, instead wholeheartedly supporting emancipation (source: wikipedia page).
For his more direct impact on the Boston area, he is a bit (read: much) more fitting a statue for the Commonwealth Mall than
It should also be noted that Wikipedia claims that as a boy, Garrison sold lemonade (though I cannot confirm this). To all you lemonade-stand-entrepreneurs: you are in good company. Unless, like me and one of my good friends, you tried to sell lemonade on a dead-end street.