Set boosters are the go-to for players who love to open boosters just for the fun of seeing what they'll get. Just like Collector Boosters are for collecting and Draft Boosters are for drafting, Set Boosters are for opening packs to see what's inside.
At least one foil in every pack:
These can be any rarity, so this is one of several places where players can get a rare or mythic rare in a single Set Booster.
Chance of multiple rares:
Between wildcard slots, the rare slot, and the foil slot, there a several ways to get more than one rare in a Set Booster.