It's always better to get a good photo than to try and fix it after in Photoshop. But sometimes, shit happens and your image is dark. Here's a great example
We shouldn't have taken this photo from this angle, because of the shadows on everyone's face. Ideally, we would have asked everyone to turn around and taken the shot from the other side. But the client wanted a photo facing this way, and it wasn't the right moment to explain why it's not going to work. So we took it anyway.
If you look closely at the image on the left, you can see the deep shadows not just on peoples' faces, but look at the guys' suits--look at the blackest parts. You can't make out any details in the suits. If you look at the brightest parts of the image, particularly on peoples' faces, you'll note the light is harsh.
Again, in a perfect world we'd flip everyone around and fix most of the problems with this photo, but here we couldn't, so what are our options?
A photo like this isn't perfect, but that's ok, we can still use it, and that's what counts in the end.
Because modern full-frame DSLR's are so good, we have a lot of dynamic range, otherwise known as "wiggle room."
And here's what that photo looks like after we've lightened it up. It's never going to be perfect, but if you printed that in an album it would look still look great.