Answer
A Default Life Time value extends a report's expiration date in the Archive by that many days from when it was generated — but a value of 0 specifically means the report expires on the very same day it's generated, effectively removing it from the archive almost immediately. This has caused real operational problems in practice: for example, a Customer Order Invoice report left at a Default Life Time of 0 caused users to get a 'result key does not exist' error when they later tried to reprocess or resend an already-generated invoice, because the archived copy had already expired the same day. Whoever configures the environment (not necessarily the end users who create ad hoc Business Reporter reports) needs to deliberately decide an appropriate retention period for each report type — leaving an operationally important report like a customer invoice at the default 0 is a common, easy-to-miss misconfiguration.