When creating a new custom-built, cloud-native application designed to be horizontally scalable, a Platform as a Service (PaaS) is a must. PaaS provides a platform that includes development tools, runtime environments, and infrastructure management, allowing developers to focus on building and scaling their applications without worrying about the underlying infrastructure. It provides the necessary framework for developing and deploying cloud-native applications with ease and scalability. Horizontal scalability, in particular, is often achieved through features provided by PaaS platforms, such as load balancing and auto-scaling.