The role of being a charge may be a self assignment, generally if someone is new to exploring BDSM and decides they would like a protector, or it may be at the order of a Master to a Slave to be someone's charge while the Master is away. Alternately it may also be a slave who has recently become unowned and has a caring or trusted friend or leather family member to serve as their protector during a period of transition.
The role of a protector does not equate to being someone's Dominant, Play partner, Trainer, or sex partner. It is important to determine that attaining one of these positions is not a hidden motive of a potential protector as it is not entirely uncommon to have predatory individuals offer to protect vulnerable explorers of BDSM as a ruse to gain access to exploit them. If you are the one to select your protector, be sure that a protector you select is someone that you can trust not to take advantage of you.