Relativity isn't a Drupal module, distribution, or install profile. Simply put, Relativity is a collection of system building methods that are specifically designed to demystify, simplify, reduce size and speed optimize Drupal.
I've been building websites for quite a while. I was around when database driven websites were a new thing. I remember the saying "Content is King". Relativity ensures all content interfaces offer consistent and simple system interaction.
Because something is simple, doesn't mean it isn't effective. Simplicity makes things less expensive, delivers higher performance and if done right, offers a personalized elegance. Every website should have its own brand of elegance and simplicity. Relativity was designed for this purpose whether in a small site or a huge enterprise.
Some websites have one content editor, some have hundreds. System interactions should be automated based on the logged in user. Drupal and Relativity work together to ensure that users are given the options allowed and never see things they cannot do.
I think too much emphasis is placed on the layout and design before the foundation of the system, its content. Relativity ensures that content always remains in the forefront.
Drupal core and a few contributed community modules can do amazing things. Custom modules make sites version dependent. I stay close to Drupal core with no custom modules.