Reach and frequency are important metrics when choosing a campaign objective.
When the goal of a campaign is to show your advertisement to as many users as possible, its campaign objective will be a reach campaign. When the goal of a campaign is to create retention around a product or brand, its campaign objective will be a brand awareness campaign. Retention will be built around someone seeing an ad for the same company/product multiple times, meaning the ad will want to have a high average frequency. This in turn will help make that company or product recognizable.