The ancient biography as a literary genre is distinct from modern biography in ways that are important for historians who use it as a source. Plutarch's Parallel Lives, the most important surviving collection, pairs a Greek with a Roman figure and draws moral lessons from the comparison. This moralistic purpose shapes everything about what Plutarch includes and excludes.

Plutarch is interested in character — what kind of man was this? — rather than historical reconstruction. He will include an anecdote from a dubious source because it illuminates character, and omit a well-documented event because it doesn't. His use of sources is promiscuous by modern standards: he draws on the same event from multiple contradictory accounts without resolving the contradictions, because his purpose isn't to establish what happened but to illuminate the moral significance of events.

Suetonius' Lives of the Twelve Caesars is organized differently — thematically rather than chronologically — and reflects a different purpose: it's a kind of systematic character study organized around the virtues and vices the emperors exemplified. The result is that Suetonius gives us extraordinary detail on certain topics (physical description, sexual behavior, personal habits) while being almost useless for understanding political events.

The problem both authors share is that they're writing about figures who died generations or centuries before them, relying on earlier sources that are often lost. Their reliability varies enormously by subject: Plutarch on Greek figures of the 5th-4th centuries BC is working with much thinner and less reliable material than Plutarch on Roman figures of the 2nd-1st centuries BC.