An ads library is a shortcut to what the market already validated. Instead of guessing which angle will work, you start from the videos that collected the most views or ran the longest in your category, and you study why.
A simple routine works well. Filter on your category, sort by views, and open the top twenty videos. Note the hook of each in one line, the length, who is on screen and how the app is shown. Patterns appear quickly: an angle every winner uses, a format nobody has tried yet in your niche.
The goal is not to copy a video. It is to understand the idea behind it and to film your own version with your app, your proof and your creators. Days running is a useful signal too: an ad that keeps running for weeks is an ad that pays for itself.
Keep a swipe file of what you find: the hook, the angle and a link for every video worth remembering, tagged by category and network. Review it before each new batch of ads, and come back to the library every week, since it is refreshed every Monday with the newest TikTok and Meta ads of each market. After a month you have a map of the angles that work in your niche, and a short list of creators to brief.