Of course it's possible to use Reason (or any other DAW) and create great music without an external midi controller. But a midi controller can really speed up your workflow with mappings to Reason's instruments, settings and parameters. This allows you to spend less time clicking around Reason's interface and more time getting creative. A midi keyboard is also ideal for live playing ideas, experimenting with progressions and quickly laying down these ideas in Reason. You could play chords or program drums on your computer keyboard, but it's only really possible to play one octave, without any variation in velocity, so you'll have to click around in Reason and manually edit your midi tracks to give your input a natural feel. Overall, it's fine to use a DAW without a midi controller, but in most cases, as you get more comfortable with the software, you'll want to purchase a midi controller to improve your workflow.