It's still not unlimited in the strictest sense of the word, but you do get more for your money. Premium gives you 50GB of LTE hotspot use at LTE speeds instead of the 15GB you get with Plus, and you won't face a cap on LTE data when traveling to Canada or Mexico.
Sprint's strategy is fairly clear: it's hoping to both upsell you to a pricier plan and, crucially, give you a strong reason to stay with its service. Jump to another carrier and the costs of access to Prime, Hulu and Tidal shoot through the roof -- you might stay simply because it's too expensive to leave.
Even so, it's potentially a better deal than at other networks if you're looking for discounts on third-party services. Verizon (our parent company) only provides six months of 'free' Apple Music service and 500GB of its in-house cloud storage. AT&T focuses on bundling its live TV service and gives you a 'perk' that can include streaming music. T-Mobile, meanwhile, is sticking to free Netflix as its hook. So long as you don't mind Sprint's particular mix of services or its local network quality, it might be the better pick if you were going to subscribe to Prime no matter what.