Kobo is all about books, book consumption and social reading experience. It's not really about the hardware as it aims to be open platform (heavily embracing Apple ecosystem solely through apps, driving Android use through Samsung and other vendors) ... its about bringing a new reading experience to the table.
Rakuten, as noted, doesn't have historic "book vision" but seems very focussed on e-vending. It's an interesting match-up and certainly can't harm Kobo's international ambitions. And, from the sounds of it, it has deeper pockets than the Indigo group does which is critical at this stage of Kobo's development.