mono backing can provide a degree of grip between your spool and the line for correct drag. this is debated by some if necessary but i have actually seen some slipping issues further into a spool in heavy big-gun drag situations. i (personally) always spool some mono first.
mono is gonna be significantly thicker than braid at the same test, so use mono to help fill up your reel as not to need 2x the length of only braid - unless you want extra length.
for example, for big-gun tuna reels i have mono backing (100 yards) for base grip, then braid backing (1000 yards of length) and then 100 yard floro top shot. only need 100 yards of floor top shot and floro is expensive!