Why did she leave the light on????

The one she went for after spilling that stuff in Roat's eyes? She filled that vase with Hypo, the stuff Sam put his pictures in to develop them...turning on the light when Roat had that stuff in his eyes and shining it on him would've blinded him.

After she throws Hypo into Roat's face she runs over and breaks the light by Sam's desk, to make Roat feel blind too

If the last light source was turned off. there would be total darkness on the set until the refrigerator light turned on.In total darkness, you would not be able to see the last scene with Mike and most of the last scene with Roat.

Yeah, I understand there has to be a light source for the audience to see, but I still don't understand why she left that light on if she was going to run and unplug it.

Because if he walks into the apartment in pitch darkness, the first thing he will do is try the wall light. If there is just a little light, and that desk light is very bright, then he will come all the way in and engage in conversation, and she will have a chance to assess the situation, throw the hypo in his face, or whatever-- if he is going to be there at all, getting him all the way in means he is on her turf, and it also means she knows there won't be street light if the apartment door and outside door are opened (not that there would be much)-- but the point is to get him all the way inside before he discovers the other lights are broken.

The final light breaks because she goes to unplug it-- Roat would not know where to plug it in in the dark-- and he throws something at her, but it hits the light instead.

Hypo is fixer, not developer. It is sodium sulfite, or sodium hyposulfite. I don't know if it's photosensitive. You make it by mixing things like soda ash and sulfurous acid, or sodium hydroxide with sulfur dioxide. It sounds nasty, but it actually has a pH of 7, and isn't flammable. It's still very irritating, especially to eyes. I have no personal experience there, but I can tell you that getting your clothes soaked in gasoline, so it is trapped against your skin, and can't evaporate, burns like sin. I wouldn't want either one of them.

So the audience could see.

It's simple folks. She left THAT light on so Roat (or Mike as she knew he was now a bad guy) could see HER when she would have the opportunity to ask "are you looking at me now?". Then after hitting him with the solution her plan was to THEN run over and unplug the light.