A SharePoint Site is a container of objects which can include Lists, Documents, Libraries and Pages. A site can also contain nested sites called sub sites. Sometimes the terms site and sub site are used interchangeably although they are essentially the same thing, as a sub site is just a site that lives beneath another site. Every site has at least one page,namely the home or landing page. Additional Pages can be added to the site by users and admins as needed.
A Site collection is made up of a Top - level site and all the sub-sites below it.
Benefits of a SharePoint Sub Site
Each Sub-site has a different function from the main site. You can have sub -sites for different departments within your business, limiting access for those who do not need the information from the other department. Each sub-site will have information which is not needed elsewhere within the company.
Basically you want to use sub-sites when ever there is a "Division" of information, you may have a sub-site for management where the admin employees are not able to access it.