This is a tradition originating from the demoscene's roots, where small demos were distributed along with cracked software, usually computer games.
Graphic artists (or graphicians in scene lingo) create the visual coherency behind a demo, which include still pictures, design elements, fonts, colors, 3D objects, textures and animation.
Musicians are responsible for the composing, arranging, mixing and mastering (and in some cases, performing) the soundtracks and sound effects in the demo.
However, tracked music and other specialized formats still continue to be used in size-restricted intros as well as demos written for more restrictive platforms such as mobile devices and vintage computers.
This type of masquerading is often just performed to confuse naïve newcomers to the scene; while some fake identities are never revealed for their time of activity, in most cases a large portion of unrelated people are aware of the real persons behind the spoof.
Though they are not strictly "demogroups" by definition, the scene always had various individuals who excelled in all areas of demomaking; rather than gathering various other persons to aid them in a creative process, they do all of the programming, music making and visual art themselves.
One-man groups are usually treated with a mix of respect and curiosity within the scene, because they're not an extremely common phenomenon, and some of the scene marks them as an unnecessary effort for virtuosity, largely because it is virtually impossible to perform above average in all areas - one-man demos and intros usually bear obvious lacks in given areas, depending on whether the creator was more a coder, graphician or musician, who adapted other skillsets later.
Perhaps the most successful one-man army of the demoscene is Dmitry "AND" Andreev, two-time winner of the Assembly demo party in the 64k intro category.