For a full, lush lawn.
Turf is pretty resilient and can rebound from dormancy and most damage on its own with improved conditions over time.
However, when extreme summer temperatures cause severe drought stress and encourage destructive insect populations, the damaged turf may need some help. This is also true of other types of severe damage.
In these cases a one-time or yearly overseed is beneficial.
Fall is the optimal time to overseed in the Pacific Northwest. It’s important to overseed a couple of weeks prior to a hard, consistent freeze. The seeds can then overwinter and germinate in spring.
The next best time is in the spring after consistent freezing but before regular maintenance begins.