Although it’s a best-practice to give every Website its own domain, Joomla! installation and database there can be special conditions in which a multi-site solution under a single Joomla! install is warranted.
An Example Application
A small business, ‘Johnson Candies’, has two separate but related brands: Red Candy and Yellow Candy. They require a single Joomla!-based Website where both candy types are visible, each with its own home page on the Joomla! site which corresponds to the domains www.redjohnsoncandy.com and www.yellowjohnsoncandy.com.
Additionally, each brand and site requires its own design: a yellow template for one; a red template, for the other.
By including both brands in a single Joomla! web installation, Johnson Candies is able to save editing time (only one login), share articles and data across both brands (or sites) and use a single feature, such as a Contact Us form, for both brands.