Impressions versus reach is an interesting way of looking at the question of how exactly your content or ad is being consumed. Ad Impressions gives you the total number of times your ad is viewed, while Ad Reach is the number of people who have seen your ad. When you take the ratio of Ad Impressions to Ad Reach, you get Ad Frequency, which is essentially the number of times each unique user views your ad.
When you track Ad Frequency, you take advantage of this data to modify your ad campaign based on current performance. With an increase in the number of times your ad is viewed by a unique user, it is more likely that the message sinks in. Generally, higher Ad Frequency can be considered an advantage to your campaign because repeatedly viewing ads can push undecided users towards buying, and further reinforce and remind users with intent to purchase.
The only case where you would want lower Ad Frequency is when your ad ends up being displayed multiple times to users with no inclination to purchase your product. In such cases, you can use targeting controls found on most ad platforms, including Facebook Ads, to ensure your ad is mostly displayed to users more likely to purchase your product.
In fact, you can use a Reach and Frequency campaign, also known as an R&F Campaign on Facebook, to predict how your ads will perform. It includes an estimation of the percentage of the audience that your ad will reach, based on factors including frequency, time, Ad Spend, and target demographic.
