a) Because there are true tzaddikim of very great stature and it is their custom to speak of great things and wonders (they performed) for they can really perform great things and serve G-d with any thing in the world, even through eating and drinking etc.; and there are tzaddikim who can perform a redemption by their eating. Because such tzaddikim exist, there are also liars who glorify themselves as also having performed similarly great things and wonders. Some of them are leaders of the generation and mislead the public as if nothing is beyond them; everything is in their hand. All this is because there are true tzaddikim who really have this power, therefore these liars copy them, as a monkey before a human, and they are called false prophets.
However, the truth is that they are doing a favour to the true tzaddikim. For there are wicked and fornicators who give charity and their charity damages the true tzaddik. Therefore it is a favour that these liars exist for, by this, these wicked people turn to them with their charity. The Holy One makes the wicked stumble that they will give their charity to these liars. As a result, the true tzaddik is saved from their charity.
b) The true tzaddik receives his holy power of speech from the givers of charity.