Fingerprint vs Face shines while (for instance) having a conversation and, because you have manners, want to maintain eye contact while pulling up a photo of that thing you are describing, or to make sure that's just a robocall and not your wife with an emergency.

It also shines first thing in the morning where your hair and eyes are so disheveled that even Siri didn't recognize you. Your thumb is probably still the same shape.

It doesn't fail as often as Face ID - you can touch the phone on table and don't have move your head to get it right (especially on the iPad). Its more secure because it requires an active action (touching) vs passive (looking).