Several points to remember:

A double taper line uses a lot more room on a reel than weight forward tapers.   Using a double taper line may mean you will need a much larger reel to get an adequate amount of backing, or have to do without.  Using a reasonable reel size and a weight forward line usually means you will have plenty of space for backing.

Backing serves several purposes:

1.  Extra line for when that fish of a lifetime runs the fly line off the reel.

2.  Fills the empty space left in the reel, keeping fly line from being wound in small circles. Small circles will develop memory over time and the line will be difficult to straighten, therefore harder to cast.

3.  Makes winding in line faster.

4.  Works hand and reel less.

5.  Makes start up inertia  less, line begins to flow off reel smoother when a big fish begins to take line off the reel.

6. Plays a part in making your line last longer by not having it in those smaller coils.