Man page contains the following info regarding -P argument:
-P Overwrite regular files before deleting them. Files are overwritten three times, first with the byte pattern 0xff, then 0x00, and then 0xff again, before they are deleted.
What is a typical use case for using this argument parameter? Secure delete comes to mind but is it really that secure to simply overwrite a file 3 times?
This post can answer to your question.
There is a technique called residual information retrieval that can read data that was deleted based on the idea that when the drive is magnetized in order to store data other parts that are close to the data is also affected by this and it should be possible to re-read data this way ... this is though a costly technique, but use it if you are paranoid ;)
By writing data 3 times (in this case) the parts next to the track on the drive should be re-set as well in order to make it impossible to re-read this way.