What is the main advantage of using site collections?

As said previously, SharePoint is all about data and data should be properly authenticated / authorized to users. By defining the structure in site and site collection, we can define roles and responsibilities according to data. For instance, in the above figure, we will assign all HR users to the payroll, recruitment, and assessment site. These users will not be assigned to the account site collection. Same holds true for accounts users.
So when you design your hierarchy of site and site collection, you need to keep in mind the enterprise hierarchy structure and design accordingly.