They serve a small number of relatively high revenue opportunity use cases (SMB/Mid-Enterprise). If you use a lot of specialized CDN and want solutions engineers to work with, know you need it but don't have the capability to do it well in house, etc, "hosted" companies like Fastly, Aiven, Elastic make a lot of sense (things that are easy till they're not). My biggest red flag for fastly would be: cloudflare.