While in a more remote area bike lights let you see where you're going, in a city there's usually enough light around for that. But you still need lights, so other people see you. [1] Unfortunately, lots of people end up biking without lights: it's easy to forget them, have them break, or end up out after dark when you intended to be home sooner. Batteries and LEDs have gotten so cheap, however, that you can get little be-seen lights for $1/each:
They don't last all that long and they're not rechargeable, but they do work for their key purpose: glowing brightly so people can see you. I've bought a bunch of them so I can give them out when people are leaving our house without lights. And they're good as backups if your main lights break.
[1] I recently almost hit a cyclist when I was pulling out of a
parking lot onto a busy street at night. While it didn't help that
they were biking on the wrong side of the road, I think the lack of
lights was the biggest issue. I did notice them in time, but I came
closer than I'd like to pulling out in front of them.