My personal view is that, if you start on the PIP and if they say your performance is not on par, they can tell you to leave the company. It's equivalent to being asked to go.
Versus, resigning on your own accord (even if it is due to poor appraisal), it is done out of employee free will. And in the HR record, there isnt an "official" record of you being "managed" on your performance.
Even though you are right to say the end result is the same (i.e. eventually leave the company), one is done out of free will, the other is being asked to leave the company. Quite a big deal of difference to me.