Many game genres and franchises first made their appearance on DOS. Examples include first person shooters (Doom), point and click adventure games (The Secret of Monkey Island) and real time strategy (Sid Meier's Civilisation). Meanwhile, Tomb Raider and Grand Theft Auto had their first releases in DOS. For anyone with an interest in retro-gaming, DOS games are certainly worth looking at.
In addition, many games are freely available as shareware or are in the public domain as freeware after the release of the source code. For example, Tyrian was released as freeware in 2004. A good source for games is the DOS Games Archive which offers freeware, shareware as well as full version of hundreds of games.
Consoles such as the Sega Megadrive/Genesis and the Super Nintendo Entertainment System ran on the same platform using the same controllers so configuring these in Retropie is straightforward. DOS games, on the other hand, ran on a variety of machines with different video and sound output. Input differed from game to game. Some required a keyboard with a mouse, some without while some would also have native joystick support. All this offers challenges to getting DOS games properly to run in Retropie. Hopefully these tutorials will help to get you started.