I back braid with mono for 3 reasons:

1. Braid will sometimes slip when tied directly to the spool.

2. If you have a whole spool of small diameter braid and pull real hard to get out of a snag, the outer wraps on the spool pull down into the inner wraps. At best you have to peel off a bunch of line and re-wrap, at worst you end up with a bad tangle.

3. Braid is expensive, mono is cheap. Braid is a lot more palatable to my wallet if I can buy 1 spool and take care of 3 or 4 rods with it.