A focal reducer will bring the apparent focal plane inwards. The ED80 was more than likely designed with it's current set-up, aka needs an extension tube to achieve focus as it can be adapted for use in a variety of ways:
As a prime focus lens;
With an eyepiece;
With a focal reducer/multiplier.
Each of these requires a differing position of the focuser and the best way to achieve this (range) was probably to make it with the focal plane just outside ...thus it needs an extension tube for most applications.
Have a look
here and
here.
Both links have calculators that can give you an estimate on how much a focal reducer will bring the apparent focal plan inwards.
