It's very useful when typing is less practical.
Driving along, can't handle your phone: "Siri, navigate to Joe's house", "Siri, read my messages", "Siri, play some lo-fi chill", etc.
Lying in bed, don't want to irradiate your eyes: "Siri, play my audio book", "Siri, set volume to 20 percent", "Siri, pause", etc.
Walking along with your earbuds in, why bother taking your phone out of your pocket? "Siri, call mum", "Siri, compose a message to Joe", etc. I agree that talking to Siri with strangers around is both rude and embarrassing, so fall back to typing laboriously in that case.
Plenty of use cases where it makes good sense, just a smidgeon of imagination required. Darned neo-luddites :)