Some people just want to get famous by getting in an election and have no intention whatsoever of being elected. Such 'clowns' should be eliminated in an election to save time and resources, and concentrate on the actual election. Also, limiting the number of candidates makes sure the votes get concentrated on fewer candidates, which means the candidate that gets elected into office has more votes and thus greater legitimacy. This is great news for the effectiveness of the government's governance.
If you only have a few choices, you are often forced to choose the "least evil" option. It might even force voters to actually research all of the candidates' positions on issues, rather than just keeping it as a campaign wad where 93% of elections are determined by who raised more money. It would work even better if we had a ranked ballot like Australia.