Well, one attraction I've read mentioned is that it lets 'lees be tickled without their reflexes getting in the way and hurting the 'lers or otherwise blocking them.
It adds to the rush. Seriously. When you can't move and you realize you're totally helpless to whatever the other person wants to do, that's when fear can kick in. Your blood pumps harder and your adrenaline starts up. It's absolutely amazing. It brings the entire experience up to a new level. For me, not being restrained in some way during play feels empty after experiencing what it can do.
For me being bound is an escape, but it's only an escape when I'm blindfolded and gagged as well. And it allows me to tap into my submissive side. I'm a ler/dom at heart, but I still like to experience the opposite side sometimes.
I enjoy just being bound too, but being blindfolded and gagged adds something extra to it and that is how I like to be bound the most. I can just not worry about anything and lay in darkness and feel tickling and pleasure.
I'm much the same way about it as Papi, in terms of the blindfold and gags, and similar to Sammi-chan as far as the rush.
For me, it's kind of a cheat, in that there has GOT to be that level of intimacy and trust to be bound, or I won't dare do it. But, if it's there(the trust), then I can just revel in the helplessness.