Being an incumbent when seeking reelection can be a big advantage. When a person is an incumbent, they can campaign by saying they have served in that office, whether elected or appointed. Being an incumbent implies to some people that this person is experienced. It also implies this person has done the job and has the experience of doing the job. An incumbent also is generally in the news more often, and this can increase familiarity of the incumbent to voters in the upcoming election. These advantages can be a deciding factor in a voter’s mind.
A good example of this is what recently happened in Wisconsin. A Supreme Court justice passed away about nine months before his term was to end. He had announced he wasn’t going to seek reelection. Three people have announced they would run. Instead of leaving the seat open until the next election, the governor appointed the candidate who is most closely aligned with his philosophy to fill the remaining nine months of the term. When this person runs in April for the position, the person will be able to run as an incumbent. The other two candidates feel this will give her a big advantage in the campaign. Running as an incumbent implies one knows the position well, has the experience to do the job, and carries some name recognition into the campaign and election.