Your site tags visitors, who you can remarket to...
When G finds someone who hasn't been to your site, but in many other regards, looks similar to your remarketing pools of people, they think those folks might be likely to also shop at your store.
So it's not REmarketing, they haven't been to your site. It's an audience of people similar to those you are remarketing to.
If 7up is the unCola, Sim Aud is the unRemarketing.
If you sell high end goods for example, can you see how this would make a ton of sense?
Of let's say you sell something that people who speak foreign languages need more often than others, like passport services, can you see how you might do better with this Sim Aud slice?
Can you see how the concept of card abandoners breaks down when translated to Sim Aud? Look at your broadest visitor lists, and certainly those who bought, but depending on what you sell, and how you drive traffic, your buyer rem lists may demographically look like your visitor rem lists... so experiment.
You might also use Sim Auds as bid modifiers, vs straight targeting mechanisms. Set up the Sim Aud for an existing Campaign, so you can see the data for this pool of people, but leave the bid unspecified. Let the data pile up, and if a Sim Aud slice outperforms the larger targeted pool, assign a larger bid via the Sim Aud.
Careful though, Sim Aud are people who don't know you, they haven't been to your site before - whereas your repeat buyers have. Be a little slow to draw conclusions when comparing Sim Aud slices to Gen Pop slices, your Gen Pops are often biased data (mix of new and repeat visitors).
Think of all the things they know about people, all of the sites they visit, their info if they sign into G services, their email contents...