I agree that the idea that BDSm is caused by abuse has some currency in the community as well as among vanillas, which is interesting since research consistently disagrees.
My own theory is that BDSMers tend to be more at home in their own sexuality than the majority, and that this includes being more than usually able to recognise and openly discuss their sexual traumas.
Therefore, even though the percentage of people with a background of abuse is probably the same as in the general population, the percentage reporting abuse is higher.
As for the question of what "givers" (tops/Doms) get out of it: speaking from my own self-analysis, I believe that the emotional reward of a BDSM scene (as distinct from physical rewards such as endorphin highs) is the security of having one partner in total control.
What I mean is that the average vanilla sexual encounter is a dance of negotiation where the participants are constantly subconsciously checking who is leading and who is following, and unless they are helped by shared social codes or excellent communication they are frequently at odds (either tugging for control, or adrift waiting for the other to give a lead), leading to low-level background stress which is too familiar to be noticed. A situation where one partner takes control and keeps it, is a relief all the more profound because one hadn't been aware of the stress till it is gone. It's no accident that the climax of almost every mass-produced romance novel is the moment when the hero takes command and the heroine happily submits.
And this relief applies whether one is the controller or the controlled, which is why I believe that the difference between Doms and subs, or between tops and bottoms, is a matter of taste rather than a fundamental divide.
In my opinion, things such as pain and humiliation come into this because, once one has discovered the joy of power exchange, one instinctively wants to enhance it by making the exchange more extreme and more obvious. And the more conventionally undesirable things the Dom/top/controler does to the sub/bottom/controlee, the more it reinforces both parties' awareness that power has been and remains exchanged. To put that in simpler language, the more I whip you, order you about, treat you as a naughty child, piss on you or whatever is meaningful for us, the more we both know that I'm in charge and you aren't. And speaking as a switch, that is the same thrill whichever end of the chain I'm on.