Letoff is what makes compound bows special. Without letoff, compound bows are just a mishmash of wheels and pulleys. But, with letoff, wow!
If you are unfamiliar with the concept, compound bows incorporate mechanical advantages to have the draw force of a bow ramp up faster than recurves and longbows and then when they reach “peak weight” the force drops off down to a much lower “holding weight” at full draw (see illustration). Because of that faster ramp up, the total energy stored in the bow is greater for a 40# compound bow than for a 40# recurve or 40# longbow, even with the force give-back from the peak weight to the bottom of “the valley.”
The most typical “letoff” is 65% but when they were introduced they were 30-40%, then 50% became popular, then 65%, and now bows with as high as 80% letoff can be purchased. The letoff percentage is how much of the draw force is taken off, so a 40% letoff would reduce the peak weight by 40% at full draw. A 60# compound bow with 67% letoff leaves the archer only holding 20# at full draw, twenty pounds! The other 40# or so has been thrown onto the cabling system so that, in effect, the limbs help pull one another.
So, being Americans, we think that if letoff is what makes a compound bow special, we want “to get me some more of that,” and the more the better.
Unfortunately this is not necessarily a good thing.
One can design a bow where there is almost 100% letoff and you would be under almost no strain from the draw at full draw. But for target archers this is definitely not a good thing. We want to have enough holding weight to get a clean launch of our arrows. Even with release aids, a very low holding weight (aka a high letoff) means that only a small force is needed to change the launch position of the rear end of the arrow. Therefore it is easier to mislaunch arrows. This is the same consideration with recurve archers who do not have enough draw force “in hand” to get cleanly off of the string. (The hardest bow I have on hand to shoot is a 10# recurve bow. Getting off that string cleanly is very difficult.)
Most compound target archers seem to have gravitated to about 65% letoff. I say “about” because exact letoff cannot be built into a bow. If you change draw weight, or brace height, or any number of things on a compound bow, you can change the letoff involved (not hugely, but some).
Hunters are more prone to use a bow having 80% letoff as they will only be taking a few shots, may have to shoot from an awkward position, have larger targets to hit, and may have to wait for a moving target to clear obstructing brush or turn for a clean shot. The extra letoff allows more time at full draw.
More time at full draw is the advantage of letoff. Because of the lessened strain on the archer at full draw, they have more time to align their bows correctly and then aim carefully. When peak bow weight happens at full draw, as it does with recurves and longbows, time at full draw is necessarily short and less care can be afforded.