The ability to store large amounts of data, for free! I have been using this service since 2014 with no problems at all so far as storage capacity goes (I've used up about 250GB total).
There are other services out there that offer similar capacities but you pay more or less money per GB stored - not worth it when they give away tons upon tonne! It's good because we can use our own servers without having to rely completely on them which cuts down costs in terms of equipment maintenance/electricity bills etc..