Yep one nerdy said it already. A footbed will generally make your foot slightly shorter and narrower, however it can actually keep your foot slightly more still inside the boot. Having the arch's of your feet filled by the footbed, can stop your foot sliding forward inside the boot.
In a full tilt there is also no insole so adding a footbed back into the liner can reduce the volume. Its also possibly to add material under the heel, if your instep is a little low inside the boot.
Custom beds are a huge importance to any boot. But its worth noting that a bad footbed could be as bad. For example I was fitting Woodsy this week. He was having problems with his boots all season, he had been to a few big shops and even podeitirsts to get the boots fitted. The footbeds he came into me were awful and they were in fact creating more problems. I remade his footbeds and it has almost sorted out his issues.
So overall go find someone who really knows what they are doing with footbeds.