Setting up a website conversion pixel will help you understand what actions customers are taking on your website, after having seen an ad. These actions could be browsing, adding items to cart or making a purchase.
You can also use the pixel for retargeting. This means you'd be able to target people who browsed on your website, but then left before making a purchase.
Given that you can't track these actions without the pixel, we highly recommend setting up the pixel before launching your campaign.