They can cause problems like making quests impossible to complete, altering your game's display, all kinds of game behaviors, your ability to play your character, and your ability to play the game at all. 'If you don't know what you're doing, using console commands can cause your game to malfunction! Executing the wrong command can cause your game to stop working normally furthermore, you may not become aware of such malfunctions right away, and you may not be able to trace their cause. In certain circumstances it may be necessary to use it to fix a bug, but it should only be used as a last resort, and a hard save should always be made prior to using. The console is a developers tool, not a gameplay aid. Just ignore them - nothing in the game requires you to do quests of any kind or at any time.
