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what is the difference between needful and necessary?
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rja what is the difference between needful and necessary?
"necessary" is used 500 times more often than "needful"
In modern usage, needful is more common in fiction (science fiction, horror) than in other genres and the two words are not interchangeable. You would not use "necessary" in this sentence:
The poor waif was needful of simple human kindness.

They used to be more interchangeable: (from 1780s):
The Constitution states that Congress shall have the power to raise funds to ... for the erection of forts , magazines , arsenals , dockyards , and other needful buildings ";

As Clive has remarked, the Indian English dialect has many features harking back to Victorian English.
Comments  
The main difference is that 'necessary' is a very, very common word, while 'needful' is, in my experience, very, verry uncommon.

I know that Indian people often say 'Please do the needful', but this is common only in Indian English.

Clive
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 AlpheccaStars's reply was promoted to an answer.
What is the difference between needful and needed
guava park 322 What is the difference between needful and needed

You cannot substitute "needful" for "needed."

Another two liters of petrol is needed to make a full tank. 

In today's language, needful is primarily used in the dialect of Indian English. It is very rare in British or American English.

Please do the needful. (Indian English) 
Please do what is necessary to complete the process. (International, American and British English)
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