Broad targeting leaves out interests and lookalikes and lets Meta's delivery system decide who sees each ad.
Why it became the default
With Meta Andromeda matching ads to people based on the creative, the ad itself does the targeting. A video for new parents finds new parents. A video about joint pain finds people with joint pain. Narrow audiences mostly limit how far a good ad can go.
What it asks of you
Broad only works with a varied set of ads. If every ad says the same thing to the same person, broad targeting reaches the same narrow group anyway. Make different ads for different buyers and let Meta sort them.
More: Creative diversity on Meta.
Related: creative diversity, Advantage+ sales campaigns.