i don't know if there's anything that "requires" them to do it per se. i suspect that most of the players themselves choose to do it for their own reasons, be it contractual (i.e. they don't have a contract at all with a company or they are currently working towards one but the terms aren't settled) or they aren't 100% happy with the gear they under contract to endorse.
i tend to think of it as something that a player may use for leverage in negotiating (why should they get paid if the company gets the publicity for free?). i do think it may be possible that the company might also be trying to decide which product to market with which player.
"Wang and Chung Wangchunged last night"- Gonzalito17