As for Greenlight's role in the storefront, this may be the biggest deal...
They'll be using it as office space, so as to move some of their back-of-store operations—"buying, receiving, bookkeeping, shipping," etc., per a press release—off-site, which will make the sales floor bigger and cleaner.
Most significantly, again per the press release, the additional floor space will allow Greenlight to have "the space flexibility to better handle large-scale projects such as the Brooklyn Book Festival, school book fairs, and large author events." (Greenlight and Greene Grape also plan to collaborate on food-book events and groups in the smaller space.)
Increasingly, local independent bookstores like Greenlight are surviving and thriving in a consolidated and digitized and illiterate world because they're able to foster a sense of community with their patrons and a sense of significance within their community. Greenlight events with local and prominent national authors—which are collegial and right at your doorstep, if you live around there—are often quite full, and bring people in to the bookstore to have a good time and see all the books on display and hear from writers (later than usual retail hours). If Greenlight has the money, and impetus, to expand—as BookCourt recently did, for similar reasons—it's an encouraging sign that Brooklyn's local, independently operated bookstores have a good idea of how to stay vital.