For me, it's the first DAW I've actually owned (I've had some experience with lite versions of others). What made me buy this, and not cubase, Ableton live, sonar etc is very simple. Reason has everything in the box and it's designed that way. Ok others have instruments, effects and things included but Reason is the only one that I feel doesn't *need* additional stuff (mostly as a result of its sandbox approach).
My PC has crashed and died various times and this has meant new hard drives etc, with fresh installs of countless instruments and effects. It just takes too long. It's soooooooo frustrating to open up song 678 and get a "plugin XYZ could not be found" popup just because you went mad and wrote every new song with a different eight synths and twenty new send effects

In contrast, I know that won't happen with reason (my intention is to resist every single refill and rack extension that doesn't come bundles with it, and concentrate on making music with what I have and getting the most out of every included bit of kit).