The mantras will be helpful to attract the universe to give what you are trying to manifest, however for example if you are blindly reciting mantra without action is not going to help (as they show in movies). Say you are reciting mantra to attract $10 Million and constantly buying lottery tickets and intensely trying to manifest it (you have a chance). You see, the intensity and belief is the key and the mantras power and channel that intensity to manifest as desired.
Note: just because I did not refer to an authentic source doesn't mean I challenge our Hindu texts, if I find it, I'll come back to update it here.
Note to moderators - Respectfully, I do not see how much I could back my general knowledge by authentic sources. It is important to understand that Hinduism or in fact any other religion was not born out of authentic sources. Let me put it this way, not every answer comes out of a book. However, we could always validate if that answer makes sense, does it serves the purpose. So I request moderators to re-consider before marking my posts for delete. Also, for the practices that were passed on from my ancestors needs no citation, Thank you.