It boils down to readability (not only text, but all controls also). While in theory high contrast is desirable, in practice contrast can be too high, and reading pure white text on pure black background is harder than reading dirty-white text on dark blue background - too high contrast kind of blinds the eyes.

Wouldn't that be an indication that your screen brightness is turned up way too high, rather than a problem to be solved by making the background brighter? I'd think that if white text on a black background was blinding, you'd really have problems bringing up a page like HN which has a solid white background...

When the brightness is turned too high, sure. But this phenomenon is not exclusive to screens with backlight, nor to very bright things.

Think about the street signs, where readability is very important. Vast majority of the signs have green or blue background, exactly because of this reason. (Sure, there are some B/W signs, but they are usually quite rare)

One other, less important reason: if you want to have a drop shadow, then your background had better not already be truly black.

This. Same is true for white, you really want a 'broken' white (very light gray) so there can still be hilites.

Even weirder then that the vim theme I'm using has a "light black" text background, slightly darker black background in the statusline, and then solidblack border around all this... Though that may be because I have my terminal background set to black rather than it being part of the theme?

Because pure black doesn't exist in nature and usually is perceived as jarring. Common design principles therefore advocate for using a colour that's almost - but not actually - black.