COMMERCE Associate With (on checkout)

I am setting up a membership site, using products as memberships with privileges based on memberships.

Please tell me what Associate With does?

For example, I have MEMBER GOLD, PLATINUM, SILVER

But i also have Add-Ons to push banner ads, or thumbnail ads, things like that.

So what happens when somebody Associates a product with another product?

Why does the system give the option to NOT associate?

Trying to understand and learn this feature.

Thanks all,

~saraJ