Every forecast question, such as "what will close this month?", is answered with the close date. Revenue reports group deals by it, the forecast tool rolls deals into periods by it, and many dashboards filter on it. A deal with no close date, or a wrong one, falls out of the period it belongs to.
While a deal is open, the close date is a prediction. Reps set it when they create the deal and should move it as they learn more. When the deal moves to a closed won or closed lost stage, HubSpot typically updates the close date to the day it closed, which keeps historical reports accurate.
The most common problem is close dates in the past on deals that are still open. These deals are effectively forecasting revenue that should already have arrived. They make the current period look emptier than it is, inflate overdue numbers, and hide which deals are actually at risk. A similar problem is deals with an amount but no close date, which count toward pipeline totals but never toward a period.
A simple routine keeps close dates honest. Each week, reps review their open deals with a close date in the past and either update the date with a reason, or close the deal as lost. Making the close date required at deal creation stops the blank ones at the source.