Unless you are running a large-scale poultry operation that crams birds into too small a space to avoid pecking problems, there is absolutely no benefit to debeaking. It is where they cut off about half of the top beak, so that birds can't pick at each other and cause injuries or death. Yes, I believe it is very painful and totally unnecessary if you are providing your birds with adequate space. It also causes added stress to the hatchling and probably increases the risk for losses during shipping.
I hadn't heard of decombing, but I would imagine it is where they remove the comb. I guess it would have the benefit in a huge commercial operation by removing a target for other birds to pick at (or as TrystInn stated to facilitate protrusion of the head through the bars of a cage) but it probably has no significant benefit for a properly managed backyard flock. You could probably make the argument that decombing would eliminate the risk of frostbite, but it still seems like a procedure that would be better left until there is an actual problem (ie, the comb gets frostbite).
