Exposure is technical, it's a number, it's not alive. Lighting is design, creativity, ratios, nitpicking, feeling, painting...
Your camera has tons of tools to help you get the image you want, but you need to keep in mind that the camera is not alive, it doesn't know what you want, and it exists soley in the world of technical, and you have to use it as such, a tool. Looking at your camera's monitor, you might see that the exposure bar is right at 0, indicating the scene is not too bright, and not too dark. If you've only ever shot based on that bar, then you need to understand that that bar is not a design bar, it's an exposure bar, and just because it says the scene is properly exposed, doesn't mean it looks good.
When you think of a great scene, you don't think that it's got great exposure, you see how deep and detailed the lighting is, how it sets the mood and gives an enormous amount of texture to the image. You need to think about what the lighting should look like for your particular shot. Think about it this way: you have a location in an office with rows of florescent lighting, all very dull and even... But you're shooting a film Noir which is supposed to be contrasty and sharp... Do you use the lights as is because your exposure bar is zeroed out?
I had quite the same situation not long ago, shooting a Noir and had a doctor's office scene, and it just had overhead florescent lights. What i ended up doing was twisiting most of the bulbs to turn them off, and keeping a few on in the areas i wanted them to illuminate. In addition i used a lowel pro light(a sharp light) in the top corner of the room pointing across, to simulate a night window. (it was a window-less room) I didn't want to make it too contrasty because a modern day doctor's office has a typical look, and i kept elements of that look with some of the existing practicals and Noir'd it up by adding a supposed window(or even just a lamp) off screen.
Let's assume your doing it all yourself, otherwise you should have a gaffer/lighting technician/other experienced person who could guide you. When you go to shoot something, anything, the first thing you should do is take note of the practical lights, how everything looks as it is. Now, if the practicals are good enough, and you can find the right positioning to make it work, then yes you can shoot without (extra) lights. But go back the last paragraph, even though i could get exposure with the practicals, it would have looked like crap, but i didn't forgo the practicals altogether either, i used what i could of them to keep some of the original office's elements in tact.
When in a more run and gun situation, a always keep at least a small lighting contingency, i expect the practicals to be substandard or unusable and bring along at least something to help the situation. This is where battery powered LED lights are just brilliant. I use the Z96 and CN160 lights and I've gotten away with murder on several situations with them. The old saying holds true: It's better to have it and not need it, than to need it and not have it.
The potential issues with practicals
The first of course is design, it's rare that a scene just works perfectly as lit. It does happen, but you shouldn't be planning for or expecting it. It's the difference between seeing it as a chore vs a desire. Desire to make it as good as you can, don't settle for the easy way.
Next is light quality. Watch my light types video to see what i'm talking about. Basically, some lights produce some terrible quality, and you should know what those are and how to look for them. The most typical are street lamps, which frequently are sodium vapor, not too pretty. Some lights can cause a flicker, and sometimes that flicker can be fixed with a 1/60 shutter, but sometimes not. Some lights can have wacky color shifts, which you might say that you can fix with a white balance but no no no, white balancing is going to change your entire shot's color, and if there are more than one type of lamp in the scene, you'll be robbing peter to pay paul (did i, a jew, just make that reference?)
The most impractical of practicals is just that they exist to begin with, and sometimes that can mean you have to deal with them no matter what.
So what can you do if a lamppost is ruining your shot?
The first and simplest solution is to see if you can just move away from it. Get away from the problem.
Can't move away? Flag the sucker. This can get impractical if it's a large scene, but let's say it's a small, short scene and you're going to mostly use mediums/closeups, a flag placed between the crap practical and your actors would do the trick. You might still be able to get a wide shot if you plan your angles with care, or just get a really wide angle so the lighting on the scene isn't as apparent. Not perfect, but sometimes imperfect is better than nothing at all.
The last and most pertinent to the theme of this article: Try and overpower it! Here's a situation where having powerful lights of your own can be of huge value. The brighter your light is, the lower the exposure of that annoying practical can be.
A combination of all three methods can yield the best result. Flipping it to daytime with a bright sun, someone holding up a silk to diffuse the light on an actor is what a low budget/run and gun shot can get away with with ease. If you're wondering why not just let the sun hit them directly, it's for two reasons: first is that it's a super hard light and will cause reflections/hotspots/actor squinting/etc... The second is that depending on your background, perfect exposure for your subject might be a few stops different than the background, and silking can help you find the right ratio so you don't end up with a blown out background.
To sum it all up
Don't look at lighting as just getting an image that isn't over or under exposed, look at it as a painting. What is the mood, what is the feeling, what elements do you want to highlight and which do you want to drop into shadow. If a location works as it, great! But keep the mentality that you want to light, not that have to light. Because it's the same as saying you want your film to look good vs wanting just to get it done.