Please teach me why "it" is needed in this sentence below.
ex: "Is it worth it to buy such a jewel?"
We don't know whether "it" is needed because, without context, we don't know what "it" refers to.
If "it" only refers to a specific thing
like paying a large amount of money, "it" is needed.
A...I think I would have to pay at least $10,000.
B...Is it worth it to buy such a jewel? (here "it" = paying that much money)
Without "it", the sentence should be as Wuisi suggested, "Is it worth buying such a jewel?'
This meaning is like 'Is it worth having
such a jewel?' and considers other things besides the money you will spend. For example, it may also consider whether you have the appropriate clothes to wear the jewel with and how many chances you will have to wear the jewel.