I'm curious how this works in practice. In my field, outright firings are very unusual - either someone has tenure, and you're stuck with them, or it's a matter of not renewing their contract.
I can think of lots of very good reasons for doing this, though.
There are some jobs where an employee is not, by policy, told that they are about to be let go until the moment it happens - generally jobs where they could sabotage things out of pique (handling finances, for example) or where there would be issues of industry security (taking client info with them). In those cases, waiting to post a job listing until after an employee has been let go would be foolish on the employer's part.
And in general, people who have been told that they are being fired for poor performance tend not to improve in performance after this is announced. They aren't going to get a good job reference anyways, so there's a strong temptation to do as bad a job as possible. For customer service jobs, for example, this could damage the businesses - customers faced with a surly and incompetent employee out to make trouble are not likely to come back. There's also the possibility that a person who is told that they're being fired will walk out immediately, leaving the job undone.
Having a restaurant or catering position filled by a vengeful (rather than just incompetent) employee strikes me as a bad idea for your business.
For jobs that have to be filled at all times, I think this would be necessary. It takes time to hire someone - post the job notice, wait a reasonable amount of time for applications to come in, read the applications, schedule and conduct job interviews, make a decision, make an offer, maybe make another offer if the applicant has already taken a different job. Some positions can be filled by basically hiring an applicant off the street, but the more specialized or skilled a job is, the longer it can take to find the right person. Starting the process only after firing the employee could seriously hamper the business.
I agree with PPs that except in extreme cases (dangerous behaviour or legal impropriety, for example) poorly performing employees should get feedback and a chance to improve before being let go.