There's the practical consideration that a properly tied-up lee can't fight back, which means that the ler is freer to relax and set their own tempo.
Then there's the psychological factor -- feeling helpless when tied up makes some lees grow more ticklish. Of course, the psychological effect can also go the other way -- for example, I personally become less ticklish when tied up, because it puts me into a more relaxed submissive headspace.
Bondage certainly has its uses for tickling. But it really depends on the people involved and the desired experience of the tickling play. There's equally something to be said for the more primal experience of bondage-free tickling.