Once issued, an addendum becomes part of the contract documents, and bidders are usually required to acknowledge each one on the bid form; ignore a material addendum and your bid can be ruled nonresponsive. Late addenda are a constant source of last-minute rework, since a single revision can ripple through quantities, pricing assumptions, and scope.
Addenda are also how an agency keeps every bidder on equal footing: rather than answer one contractor's question privately, the owner publishes the clarification to all planholders so the competition stays fair. When a significant change lands close to the deadline, agencies will often extend the bid date along with it — but that isn't guaranteed, so the safe assumption is that the clock keeps running. Monitoring addenda is core to what Nonlinear does: it tracks new addenda as they post and compares revisions against the prior documents, highlighting exactly where a change affects your numbers so nothing slips through in the final 48 hours.
