As others have mentioned, dark themes are easier on the eyes. A white background is similar to staring at a light bulb -- definitely not something you want to be doing for hours every day.
The dark themes have a bit of a nostalgia value as well. I remember learning to program on an Apple IIe, where all you had was a black screen with green text.
If you're used to coding with light themes, it may be a little distracting to switch to a dark theme at first, but after a few days, you'll start to see what everyone means by the eye strain benefits.