A humped back can make it harder on the neck if riding very aero and low. The straighter the back, the less the neck has to be craned upward to see ahead. Perhaps there are other things too. But this is the biggie for me straightening my back while riding. But it's something I have to keep reminding myself about as I'll often find my neck craning up to see and my shoulders and back humped up.
But when I remember to straighten it out, then thing are easier on my neck instantly. I see some pro riders with humped backs riding grand tours. So apparently it's not necessarily a issue for everyone. Or else they as do I, find it hard to hold that straight back all the time.