Why does it make sense, it tells them that their performance is not up to scratch and gives them a chance to improve and do the job they were employed to do. I agree with Qld
IR Consultant though, there is not set number of warnings. You just need to be able to to show that you've been fair and reasonable in your requests. This generally includes advising the employee first in a meeting and then confirmimng in writing of the areas in which their performance is not meeting expectations and given them time to correct their performance before terminating them. I would generally discuss a review period of 2 - 6 weeks depending on the issues.
This also gives the employee an opportunity to let you know of any factors (either personal or professional) that are effecting their work. A couple of months ago I had a classic person not performing, turns out he had a gambling problem and once we had let him know that we would support him in getting some help there has been a marked improvement in his work. Either way, in a perfect world the manager would have been discussing this with them at least on an informal level before it gets to this point.