shortfuts is a free extension and always will be. shortfuts premium is a way to support the developer and unlock a few features that may make shortfuts even more beneficial to you.
Without shortfuts premium, there will be a small delay when using the popular "Buy Now" shortcut. The delay may only be less than half a second, but it might mean missing out on that great snipe!
Without shortfuts premium, you'll be interrupted with a popup if you issue too many searches within a minute. It's not a big deal, but may interrupt you're sniping!
With shortfuts premium, you're able to press a single key and fetch the most recent BIN prices for a player on all platforms and get shown that information directly in the web app.