I like the characterization that Andrej Bauer uses: "while relates to goto, as effect handlers to shift/reset" :-)
That is, you indeed need delimited continuations to implement effect handlers, but they have more structure than, say, shift/reset. In particular, instead of being able to just yield (shift) with any function to an innermost context (reset), you can only perform a certain set of operations that are handled by a function defined in the handler.
This gives various advantages, in particular,
- you can give simple types to effect handlers (while shift/reset for example needs an answer type system), and you can use fine grained effect typing (instead of just a "shift" effect, you have "<exn,console>" effects)
- you can do better reasoning as the set of operations is restricted
- and (as a consequence) you have more opportunity to optimize them. In particular, tail-presumptive operations can be implemented to execute "in-place" without capturing the continuation for example.
- finally, different effect handlers are compositional and can be combined freely (unlike shift/reset; for example shift aways shifts to the innermost reset so we cannot compose arbitrarily compose with multiple resets).