This blog has spoken before on how promotional key tags are both functional for the consumer and useful for a producer. Let's focus a little more on the most evident use of a tag, keychain, or other similar designator: keeping personal things identified. Putting a tag on a purse, bag, suitcase or other item reduces hassle on plane trips and makes it easier for the user to extract their particular luggage from a pile. With this sort of application in mind, promoters can think ahead when picking out what kind of tags to go with.
For instance, durability is a top concern, as is visibility, whether it's a bright color, specific logo, or shape. The latter can be particularly useful, and chances are some of the most memorable attachments you recall from your own life stick out because they looked like specific things, perhaps a company mascot or related image. It's also important to consider exactly how the tag in question will be attached to the greater item. Loose chains, commonly seen on key rings, can be easy to use but might not be the best fit for a heavy-duty suitcase handle, especially if that suitcase is going on a long trip. Likewise, custom key tags that attach themselves via plastic strips can seem more immediately durable but aren't as flexible as a chain. It all comes down to what you see your audience doing with your product, and who that audience actually is.
Personalized key chains have a lot of things to offer, including that most basic thing: standing out in a crowd. How a company does that is up to them, but regardless, it's an easy thing to provide and one a customer could respond to strongly.