When traveling by air and checking bags, some airlines will let you declare an item as fragile and put stickers on the box or bag. How common is this policy? What do the fragile stickers do (i.e. how do they change the baggage handling process)? Is this generally recommended when traveling with fragile luggage (to avoid items being damaged in transit) or discouraged (because it may make an item a bigger target for theft)?
Think about what your luggage goes through from the time you check it until you get it back.
It travels on automatic conveyor belts. In many situations, it must be shunted from one conveyor belt to another. This is done by machines, not humans. The machines cannot see the "fragile" tag.
It is moved from conveyor-belt to dolly by a human. The human may, or may not, notice the "fragile" tag.
It is packed in the cargo hold by humans. Depending on the orientation of the item, the human may, or may not, notice the "fragile" tag. When they pack it in the cargo hold, they may put other bags on top of it.
It spends some time outside, exposed to the elements. No matter how many fragile tags you put on, there will come a point where your suitcase must make its way up the conveyor belt into the aircraft. If it is raining, snowing, cold, or hot, it will be exposed to the elements at this point.
It spills out of an automatic conveyor line onto a rotating baggage carousel. As it rotates on the carousel, other bags may fall on top of it. Other passengers may pick it up, thinking it's their bag, and handle it. When they discover it's not theirs, they may, or may not, honor the "Fragile" sticker.
Oh, and let's not forget:
It flies through the air, at 40,000 feet, at 600 miles per hour. It may encounter bad weather or turbulence, causing intensive shaking of the contents for a prolonged period of time.
When all is said and done, it's a miracle anything gets anywhere unscathed, "fragile" sticker notwithstanding.
As an airport security staff member I can comment and advise on this topic:
'Fragile' items should be taken to your designated ‘CHECK-IN desk’ where they are usually weighed and directed to the alternative 'out of gauge' screening facility, often used for declared genuine fragile items such as very large passenger cases, bike boxes, large tool or camera boxes, gardening items, machinery, wheelchairs.
All items going on-board an aircraft are subject to the extensive x-ray screening and inspection, particularly if prohibited items appear to be contained. Security staff will require your presence while they inspect the contents.
Departing airports are generally careful with all luggage items. However, the destination airport are only required to throw or lift items onto a in-bound conveyor. In all situations you are strongly advised to carefully wrap fragile items, using bubble wrap or other protective material inside a strong outer case. Applying fragile stickers will improve the awareness but not guarantee safety of your property. Additional insurance is very recommended should the case or valued contents be damaged in transit.
Airlines provide full Terms & Conditions which you are also advised to read fully before packing and departing for your airport. Pay particular attention to prohibited items and dangerous goods.
I would say that they do nothing.
As an example, this Jetstar page explains has a list of things that you should not include in your checked bagage which includes fragile items. I can not imagine that you would find an airline that would make any sort of promise that they are going to treat your baggage gently. There would be little reason for any airline to encourage "Fragile" stickers since that just strengthens enivitable passanger complaints of damaged luggage.
(An aside is that they say these things could be "damaged" but the items suggest that "stolen" should be in there as well.)