Most artists are taught very early on that if you want to create something and make it look convincing at all, then you need reference material. Doesn't matter if it's stylized, or in general isn't supposed to be photo-realistic you really should get some reference material. It is however, I feel it's something that doesn't get said to Designers often enough. Typically as a designer your job is to ensure that the gameplay always functions the way that we intend it. So when we whitebox a level we're building it specifically with the gameplay in mind. Every object you place in that scene should be structurally significant. For this, you can take some artistic liberties with the area you're creating but if want to create something you can truly call professional it should still be believable. Function trumps form but that doesn't mean that form isn't important. Assets should always be placed throughout a level thoughtfully and in a manner that doesn't make the player question why X-object is in Y-position or even why X-object is even there at all.
Reference material also helps significantly with Set Design. Set Design for those who don't know are all of the parts of the level that makes the level feel like an actual place. As though people actually lived there or time has had some sort of effect on the area. Even games like Portal, which is theoretically in a relatively sterile testing environment has Set Design and in fact has amazing set design. Though the areas in Portal are unlike anything you might find within the real world, inspiration can still be drawn from areas in the real world to help develop it.