This is a very good question. When you install the Google Remarketing Tag onto your website, you will be able to store customer visits and collect them as 1st party data. You can then remarket ads to those people. But those customer visits only count if they clicked through an ad. As such they are not a holistic view of all the traffic on your website.
You may also be getting traffic from social channels and referral sites, etc. By adding Google Analytics audiences to your Google Ads account, you will also get to serve remarketing ads to all past visitors on your website, whether they clicked through an ad or not.