Use cases for Drupal multisite

See the original discussion at #3004496: Improve multisite compatibility with composer.

While there are various considerations that should be taken into account when deciding whether to utilize Drupal's multisite feature, here are some common reasons why site builders choose it:

• Users use multisite to create 'replica' sites for others. (All one codebase, same features and functionality, only the theme is different from site to site.)
• They use multisite to host several related sites, like a university with dozens of departments that all *mostly* need the same features, but some departments have special things (shared core, shared module 'profile', with different sites having different custom modules)
• Sometimes they upgrade a module on just one of their sites to 'test out the upgrade' so that one site has a newer version of the module than the other sites.
• Sometimes they share database tables. [historically for shared hosting where only 1 database is given]
• Updating one Drupal module for a security issue that doesn't update the DB is simpler for a multisite than it'd be for multiple independent sites [works for modules only, not core]
• Both Simpletest and functional PHPUnit-based tests make use of multisite features in order to run tests.
• Sometimes they use multisite to save on per-codebase hosting expenses either via upfront provider costs or because of difficulties provisioning and managing hosting in-house.
• Users use multisite to reduce resource usage: Running multiple sites that all share a single opcode cache is memory efficient
• Use a multisite for development. It's really quick to spin up a new subsite to work on a specific issue.
• For the hostname configuration files support.

If you need to implement custom behavior, (look at the following example) then you can just do multisite with code like that:

/*
 Some solution to make this drupal multisiting be clever.
 I want to open each my subsite as subdomain following prefix to main domain
  - subsite1.sitemane.com
 and also following subfolder
  - sitename.com/subsite1
 Below is solution howto make it to be resolved at the same time vice versa
 @see https://drupal.slack.com/archives/C1BB308HH/p1605616665086200
*/
foreach (new DirectoryIterator('./sites') as $fileInfo) {
  if (!$fileInfo->isDir()) continue;
  if ($fileInfo->isDot()) continue;
  if ($fileInfo->getBasename() === 'all') continue;
  $sites[$fileInfo->getBasename() . '.example.com'] = $fileInfo->getBasename();
  $sites[$fileInfo->getBasename() . '.hasselt-2019.docksal'] = $fileInfo->getBasename();
  $sites['example.com/' . $fileInfo->getBasename()] = $fileInfo->getBasename();
  $sites['hasselt-2019.docksal.' . $fileInfo->getBasename()] = $fileInfo->getBasename();
}