How does low competition/opposition decrease voter turn-out in election? Why is it important for a democracy to have a reasonable amount of competition?

Little opposition can lead to voter apathy in an election.  In districts that are heavily Democratic or Republican, the opposition party may run a relative unknown, who has no name recognition or publicity, just to prove that they ran someone in that election.  In that case, voters do not see any point in coming to the polls if they realize that the candidate with name recognition is going to win anyway.  In national elections, if people cannot see much difference between the two major candidates, people will stay home, because they say that neither candidate is worthy of their vote.  
A democracy should have a reasonable amount of competition, because if the voters are active, then it at least makes the winning candidate aware that there is a significant minority in his/her district that should be heard.  Also, if people turn out for a little-known opposition candidate, then a more serious candidate may run in the next election and seriously threaten the incumbent's position.  Low voter turnout may also be linked to a lack of interest in political issues; this lack of interest is a threat to democracy in that the system works best when the citizens are observing their politicians at work.