Drupal Multisite Use Cases

(See original discussion at #3004496: Improve Composer multi-site compatibility.)

While there are various considerations to keep in mind when deciding whether to use Drupal's multisite feature, here are some common reasons developers choose it:

• Developers use multisite to create replica sites for others. (All sites share one codebase and the same features and functionality, with only the theme differing from site to site.)
• It’s used to host multiple related sites, such as a university with dozens of departments that mostly need the same functionality, though some have unique requirements. (Shared core, shared module profile, but some sites use different custom modules.)
• Sometimes, a module is updated on only one site to “test the update,” resulting in one site using a newer version of a module than others.
• Database tables are sometimes shared across sites. [Historically common on shared hosting where only one database is provided.]
• Updating a single Drupal module for a security fix (that doesn’t require a database update) is easier in a multisite setup than across separate installations. [Note: this only applies to modules, not core updates.]
• Both simple and functional PHPUnit-based tests use multisite features to execute tests.
• Multisite is sometimes used to reduce hosting costs, either due to vendor pricing or due to the complexity of provisioning and managing separate hosting environments.
• Multisite helps reduce resource usage: running multiple sites that share the same operation code cache saves memory.
• It's useful in development: very fast to spin up a new child site to tackle a specific issue.
• Supports hostname-based configuration files.