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It's part of the onboarding of any Facebook Ads user (i.e., advertiser) to implement the Facebook Pixel on their site.

Without it you're not going to achieve much on Facebook as you need to feedback their system when a particular ad had an impact so that they can model the ad delivery accordingly and get you more converting users. Whatever your conversion is (viewing a page, registering, purchasing).

The Like button primarily allows Facebook to monitor your presence across the internet. Tracking pixels can be deployed on a page by page or site-wide level (included like analytics scripts) to enable remarketing or retargeting.

The site-wide implementation will, for example, enable URLs that meet %string% to be grouped into a "segment". And then you can serve specific ads to that segment while excluding others

The pixel is something Old Navy might actually seek out. They want the insights and increased ad roi that they provide.