If you can come up with a compelling Art Direction, and cool-looking visual designs, and then create assets based on the ideas you come up with in your head, then concepts are not critically important.

However, concepts can be great for marketing, conveying ideas to other modelers, proving out an art style without having to build the actual assets etc.

So, it depends on your needs and the skills you have.

I've created plenty of assets for AAA games in the past using imagination and working without concepts. However, I totally understand how useful concepts are...it's just that they are not ALWAYS needed.

The hardest part of 3D modeling is getting started, because planing is so important. Concept art gives the 3D modeler a foot hold, they can start with the basic shape even when no real planing has been done.

As the people said above, concept art it's important for teams as 3D modelers need to work fast on a large project; there are people like level designers and animators waiting for the 3D models.

However concept art is extremely useful to all 3D modelers. Making a 3D model of a image is always easier than making it out of your head, Because you won't pause to long when you finish one part, wondering what to do next.

The concept image also allows you to better understand what the lead artist or client wants, most problems happen because of miss communication between client and modeler.

If you ever hire a 3D artist, you will find that if you provide them with only one or two concept images, the model will be a almost exact copy of one of the images, even when you tell them you don't want a exact copy.

Most artist will tell you to always work with a reference. Even if it's a crappy sketch or a grainy image from google.

If you're making a coffee table or chair, find a reference on google and load it into your program. If you're making a cartoonish character, do a sketch.

The natural workflow of 3d programs is very mechanical. If you 'go with the flow', lazyiness takes over and you end up with a boxy, static result. If you're working with a reference, especially if it's something organic, you'll be forced to go against the mechanical workflow of the program to make things more organic.

There's been times where I've done freelance work where I gave them a rough blocked out 3d model as a 'concept'. However, I was doing it off of a really really crappy sketch that I made. I was just to embarassed to show them my awful sketch, and it would have taken me longer to make a better sketch than to make rough 3d concepts.

Yes, concepts and references are still important when you're working alone.

I'm assuming however, that you're drawing skills aren't rock bottom garbage, like stick figures and smilie faces. My own drawing skills are somewhat degraded, from not doing it regularly for years, but I can still slop together cohesive concepts - even though no-one would put them on their wall.

For starters, it helps you to decide if your idea is any good or not, as well as to solidify the concept you have in your mind. Sometimes the idea you have seems good, but when you actually make a sketch and step back and look at it, you realize that it's not as great as you thought. So from there you improve upon the idea, or even scrap it and not waste your time if it ends up being totally worthless.

Another benefit to concepting is understanding your concept in a more 3d-way. Personally, when I have an idea, it's generally somewhat fuzzy. I may understand what it looks like from the front, but not the back. Maybe I hadn't thought about how it moves, or the details.

Overall, the biggest benefit is time saving. If you're going to model, texture, rig, and animate something, you'll be devoting days if not weeks to it. So spend a few minutes and put it to paper. If it isn't worth a few minutes to make a sketch, then it probably isn't worth the time you'll be putting into the model.