Library > Ad library collects the creatives from ads you have actually run in connected accounts (Meta today). It is the record of what your brand has shipped, next to the Assets library of what you could ship next.
What it is for
Finding a past creative fast, by name or hash, without digging through Ads Manager.
Reusing proven creative in a new bulk upload batch.
Giving the agent context: "make 3 variations of the blue testimonial ad we ran in June" works because the ad is here.
Note the naming: this page is your own ad creatives. Competitor ads live under Inspiration > Ad library. Same words, different sides of the battlefield.