Progress reports exist for three reasons: progress monitoring, contractual requirements, and records. 

Monitoring matters because you need alarms along the way, not a surprise at project end, so you can identify true delays versus hidden risks and act with recovery measures before they grow. 

Contractual requirements shape the report itself, since most modern contracts define the high-level requirements and frequency of progress reports.

The most overlooked reason is records, because a snapshot of engineering, procurement, and construction at each point in time is often what proves who caused a delay when an extension of time claim or a dispute comes months or years later.