"What is that spoon for?"
Translation:¿Para qué es esa cuchara?
That rule has been broken so often that dangling prepositions now are consideted acceptable.
I think for the same reason you used "es" in your question "porqué lo es "es"?"
The "general is" in Spanish is "ser"
What is that? What is this thing for? = ser
I guess if I had to pick one of the criteria for the use of "ser" it would be that it's for a "description" or "characteristic"
What is that spoon for? This spoon is for ...... you're describing the spoon/asking about the spoon.
SpanishDict says SERVIR as an intransitive verb can mean "ser útil" in Spanish and "to be useful" in English.
servir para algo = to be useful for something
¿para qué sirve? = what is it for?
¿para qué sirve este aparato? = what's this gadget for?
So why not accept...............¿para qué sirve esa cuchara?
above you put the correct sentence, just like I did but in the lesson you did not accept my answer and put as correct : cucharilla which means teaspoon, so if you make such major mistakes how do you expect others to learn a new language! Why doesn't someone responsible go through the lessons and correct the rubbish the makers of the site have put in it?