Collecting useful things, like food and bedding material, is something that many animals practice and is a normal, largely innate behavior. We are all familiar with squirrels gathering nuts for the winter and birds carrying nesting materials back to the roost in spring. In addition, dogs burying bones in the backyard for future consumption is such a cliché that it's a stereotype of the species. Even we humans are described as hunter-gatherers in terms of our behavioral roots. But what of cats? Do they instinctively gather and store things? The answer is partly yes and partly no.
In nature, cats bring prey items back to their nests for consumption but they don't usually store what they have gathered for future use. They have a more immediate mentality than that. Cats usually kill and eat their food fresh, living more for the moment than the future. The frequency of prey-retrieval behavior depends upon the cat himself – his gender, inclination and circumstances.
Mother cats bring home live and dead prey in order to teach their kittens how to kill and appreciate prey as food, respectively. This is the classical example of prey retrieval by cats. Since no behavior is exclusive to one sex only, males must also be capable of this behavior, though they show little to no interest in their offspring (they're not usually around to do any parenting).
Less obvious examples of gathering/collecting behavior by cats include queen cats returning itinerant kittens to the nest and transporting kittens from place to place by the nape of the neck. The circumstance of having kittens to feed and care for encourages maternal gathering of prey and group care of the kittens. Of course, some cats are better mothers than others and will do what they have to do more conscientiously, so there is individual variation in the drive to retrieve.
In the domestic situation, depositing dead prey animals on the front doorstep is probably one of the most well known forms of feline gathering behavior. This behavior is presumably a reflection of what goes on in nature. Since cats regard their owners as maternal figures [owners groom and feed them like their moms], it could be that this behavior is more for effect ("see what I can do") than to provide food for the family. It could also be a sign of affection and bonding that a cat feels for her owner.
Carrying non-food items, like stuffed toys, to the feeding area and dropping them in or near the food or water bowl is also a fairly common behavior in indoor cats. The reason for this behavior is not entirely clear. Some people believe the cat is retrieving kitten-facsimiles while others think that a prey motivation is involved. I favor the latter theory because it explains why cats drop the toys in the feeding area.
One unusual mode of gathering behavior that makes no biological sense is the collecting and hoarding of shiny objects, including jewelry and small metal objects. The items are not just retrieved; they are stashed and hoarded, even though the collector cat seems to have no further use for them. This strange magpie-like or miserly behavior is particularly prevalent in the short-legged Munchkin breed. Presumably a gene that controls collecting was inadvertently concentrated in selecting for other traits considered desirable when the Munchkin breed was first established.
Non-Munchkin cats may also show jewelry-hoarding behavior, or show it in a different form. In one notorious case in England, a domestic shorthaired cat took to stealing neighborhood childrens' stuffed toys. The cat ended up with a pile of teddy bears and other stuffed animals under a bed in her home while the neighborhood children were in tears. Whether the cat thought these stuffed animals were like kittens, prey animals, or just items to be collected is open to speculation.
Most often no treatment is needed, especially when simple, occasional retrieval of items is involved. Cat owners simply watch their pet's behavior with interest, and wonder in amazement at their cat's seemingly pointless efforts. However, the primarily Munchkin syndrome of hoarding shiny objects may be a form of obsessive-compulsive disorder (OCD) and, as such, could indicate some underlying anxiety. If you see behavior of this type, you should probably consult with your veterinarian or a behaviorist to determine the suitability of your cat's environment and the presence of any obvious stressors, like infighting between household cats or unwelcome feral visitors outside. Addressing such stressors may help reduce the frequency of the feline kleptomania. For non-responders to environmental manipulation, the behavior might be controlled using anti-obsessional medication, though this is rarely, if ever, necessary or appropriate.
The only real problem with the behavior occurs when valuable objects are hidden. If you own a magpie Munchkin cat, you must be prepared to find jewelry missing from time to time. It will likely turn up in the cat's stash, if you can find it, but the constant loss of valuable jewelry can be somewhat perturbing. The only advantage of this behavior is that if you actually do lose a piece of jewelry, at least you can blame it on the cat, thus escaping culpability yourself.