▲ ▼ Configuring custom domains for SaaS is difficult
Mature SaaS products often allow users to bring their own domain / subdomain. This allows for customizability and familiarity for their customers. Or perhaps you need to have a 3rd-party tool hosted under your domain for technical reasons. Scaling the SSL / TLS certificates for all these custom domains is very complicated!
In addition, your SaaS might need to support white-labeling so that clients can have their own branding on your app, such as their logo, colors, and other customizations to make it appear like part of their application. This is important when your user is trying to establish a consistent brand with their customers.