In most cases I think it is a way of shaming others and rationalizing one's own unempathetic behavior.
People who like to attack and exploit others want to feel justified for doing so. So to rationalize what they're doing, they label the others "whiteknight". Busybodies who aren't really good people but only have stupid and irrational ulterior motives.
This is further shown by the fact only a certain portion of the time that the term is used, it is a man helping a woman. In many case, it is just someone sticking up for someone that another person does not like.
You don't have to think that someone is defenseless to defend or support them. People tend to support anyone they like or causes they like for a variety of reasons. People will say things like "oh, you're just white knighting Barack Obama. Obviously most of the people "white knighting" Barack Obama, are not expecting sex in return. Or think that Barack Obama is defenseless. Just merely sticking up for a president against attacks they feel are unwarranted.
I'm a "white knight" much of the time. I don't want sex from people and I don't think they're defenseless. I just don't like black knights. I think their behavior disgusting and I don't like human beings being exploited. People are not excused for going around and posting other people's docs who are innocent, being sexual harassed, or anything like that, under the rationalization of "well at least I'm not a white knight."
It is almost comical villainy how some people go on about "white knights". "Those horrible do-gooders".
No, better yet.
"Oh look what clever childrenwhite knights.
See them study, watch them learn fanboy, watch them expose trolls.
How I hate those goody-goodies! How they make my stomach turn.
I've got a little secret, that'll really make them cry.
It's a nasty kind of magic. From a special kind of black knight."