To answer this question, it's important to understand through whose lens we are judging incumbency and at what level of office the incumbent currently holds. For the sake of this answer let's assume that the incumbent is someone high-profile (i.e. a US senator).
From the incumbent's perspective, the primary advantage is the incredible amount of free publicity that comes with being the incumbent. You are already a part of the political machine and, depending on your station within the senate, you have the ability to hit the CNN/MSNBC/etc. talking head circuit to discuss either your policy views and your intentions for office, current and future—or, cynically, whatever will get people to vote for you in the next election. A secondary advantage of incumbency is that if you've made friends as a senator you get, more or less, free support from other people who share your public reach.
Conversely, and perhaps ironically, the aforementioned advantage can simultaneously be the primary...
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