It's best to delete it. Every time you turn your P3A on it sees the file and does a verision check procedure for the drone, then the battery, and then writes a log file to your memory card. Is it necessary... probably not, but it will shorten your startup time, save a tiny bit of battery and keep your memory card space free for additional photos/video files.
Worst case... it could snafu on you and run another update. Best just to get rid of it once your update is complete.
It's not necessary to reformat your card every time you dump files. Occassionally.. it's a good practice. It will help keep your file fat system defragged. If you use the memory card in another device (camera or what not) then it is a good idea to reformat it once it's back in use in your drone. Other devices "may" have written other/hidden files that "could" potentially interfere with your drones normal write operations.
Better safe than sorry and loose a whole shoot session with a corrupted file, right.