Backing will be larger diameter than mono, so you need far less of it to fill the bottom of your reel, and it doesn't degrade like mono under pressure. The same forces that make mono hard on reel spools is also hard on the underlying mono, thus you have to replace mono whether or not its been used once it's been compressed. As such, in the long-run, filling the bottom third of your large capacity reels is much cheaper to do with backing than w/ mono straight through. But, to each their own.
As a side note - high end reels are high end because the grade of aluminum and thickness/milling pattern are specifically designed to allow them to withstand mono compression and expansion. So, with high end reels if you want to fill your reel with 500yds of mono you can fly at it, with cheaper reels, you may warp or crack your spool.
Cheers!
Ukee