In some jurisdictions and situations, incumbents have structural advantages over challengers. The timing of elections may be determined by the incumbent instead of on a set schedule. For less signficant political offices (e.g. school trustee or other municipal positions) the incumbent is often the only candidate with name recognition.
In American politics, the incumbent receives a more negative view than the challengers, but incumbents have traditionally won their party's nomination to run for office (regarding the office of the president). Congressional incumbents almost always win, unless they are of the opposing party (a Democratic incumbent in Texas would have a less likely chance of winning than a Democratic incumbent in New York).