If you already have a unique list, such as a subscription list, customer list, or more, you can simply upload this list to Facebook and target it. This may seem obvious, but people often don't realize that this is an option, which is a huge missed opportunity.
By targeting people on an already curated list, similar to Facebook Pixel ads, you're targeting people who are already familiar with your product and need to be reengaged. They may have shown interest but not yet made a purchase, or may have made a purchase but not come back in a while. Using custom files to target people is a no brainer.