I get it, to sit back, but why? Are they meant for people who wouldn't fit on their frame if they had a straight post? Is there a technical advantage I am missing, more weight over the wheel? Seems like if anything, you would want to be further forward for better power transfer, not back further.....

It's just fit.

Most riders need some set-back to acheive KOPS

Most road bike frames are designed to be used with setback seatposts. Not too long ago, all seatposts were setback. Be careful of using a straight seatpost that upsets your weight balance.

OP...Its not a bandaid. Top riders pretty much all ride with a setback post and they don't ride $10K bandaids.
Its about weight distribution and fitting a larger rider to a smaller frame. Most top end bikes that come with proprietary seat posts which only fit that frame run about 20-25mm setback and it is part of the equation when being fitted for a given frame. On a smaller frame moving rider weight a bit rearward helps front back weight distribution because if you think about it...riding a smaller frame to achieve the necessary cockpit length for would have too much weight over the front wheel...not good for descending either. A smaller frame for a given rider size is both lighter with typically better standover and has a shorter wheelbase for quicker handling.

and numb hands and aching knees

Because you need room for the wrench on a single-bolt seatpost. Not about fit at all... I mean, if it were, wouldn't bikes just come with slightly slacker seattubes? It was either a setback or a cutout, and cutouts are harder to manufacture. Or you can do the whole two-bolt thing like Thomson and Campy, but these are more expensive solutions as well.

Yes, like Tom Danielson..oh wait, he rides a zero offset..with it slammed forward.

Setback is yet another adjustment to get your pelvis in the best place relative to the cranks. Some need it and some don't. Since road frames range from 72*-74* STA it is easier and cheaper to use setback to compensate. A slacker STA may call for a longer chainstay, a no-no for some people.

Chainstay would only increase 3-4mm if at all. And if this were the case, why not take the setback to a further extreme?

Yeah, I know. Are you saying people won't fuss over 3-4mm?

yikes, kops? no.

Yea, but they only start doing that if you change it. If I took your bike and increased the wheelbase a few mm, you might fuss. If you bought it that way, why would you think to complain? So why setback in the first place?

Some saddles require a different setback then others. I bought a light touring bike and put a Brooks saddle on it. Could not get the saddle back far enough (knee pain is an indicator for me). Brooks saddles don't slide back as much as others due to all of that laceing they put underneath. Changed the "radial" post out for a 2 bolt model with extended setback and life is good. I tend to ride smaller frames as my legs are long and torso is shorter and compressed due to age. So, I need a post/saddle combination that allows for me to adust fit.

BTW, whats up with those radial posts? Really couldn't stand the one that came on my Salsa Casserole. The whole saddle tilt versus setback was much too finicky for me.

I'm no expert on fit, but here's what I did and it feels pretty good. While coasting or sitting on a trainer, put all your weight on your feet in the pedals. Try to lift your butt and hands off the seat and bars so you are balanced on the pedals - kind of like when you are about to go over a bumpy section of road and you lift your weight off the seat and bars. In my case, this meant that I had to slide my butt back to keep from falling forward. A setback seatpost with my saddle puts the seat where my butt wants to be when I'm balanced on the pedals.

On my other bike, which has a Brooks, when I do this, my butt hangs off the back of the seat a little. That bike has a straight seatpost.

It serves more than one purpose. Your reason was a good one but only covers one aspect. Others have mentioned fit, saddle rail length, and STA. I wish more performance oriented road frames came with 72*-72.5* STA's but people want what the pro's ride. So we get tight short wheelbase frames that need a setback post to fit most of us.

Kops yes.

never heard of him.

slammed forward with a fairly relaxed seat tube angle and the saddle with the longest tail legally allowed.

besides weight distribution to be centered on your frame you need to take in account your femur length

and the answer is?? just fit? if so, then i can see that. ive never used a setback post, and tend to ride pretty big frames with traditional/ish geometry. maybe if i were riding something compact id see the benefit. so for the record, is a seatpost with cradle that curves back slightly (not centered with the seat post, and seat tube) considered a set back post? or are we talking major set back like those bendy thomson posts?

kops, no. kbps.

<3

I have long thighs though you would never know it by looking at me. Regardless, I can barely get my seat back far enough on most frames and could use another mm or so. Personally, I'd prefer to have the seat centered in the rails both for looks and I would assume the stress on the rails would be less with them centered on the post. Thus, a setback seat post would seem to make sense to me.

The Arione sits you at least a cm further back than any other saddle I know. He's pushing the seat forward to compensate. Probably has short femur as well.

But yes, length of femur has a lot to do with it. I ride a 72 deg STA and can just get my saddle where it feels best. Knee is a couple of mm behind pedal spindle. Could never get there with a 73 deg STA.

Old school frames, I believe, had slacker STA and longer chainstays. Modern trend is steeper STA and short chainstays. Giant advanced M/L is one of the few bikes that had 72 STA, also had a well designed seatpost where you could reverse the clamps for more or less setback. They may have steepened STA this year though.