Here is a little history about fly lines and backing as I recall it. When I started fly fishing the only fly rods available were bamboo and the only fly lines were silk. Early fly lines were woven Horsehair and then they advanced to the silk lines. The reel was sized to hold the whole line with no backing. You had to size the reel to accept the line and to have some space left on the reel. The space was needed so when you wound the line on fighting a fish you would not rub the line on the reel housing. Silk worked OK but it was no way close to the toughness of the lines today. The line designations were by letters designating the size of the line. A double taper line for a 6 weight bamboo rod was HDH The H was smaller in diameter that the D. The lines did not have the sophisticated tapers that we have today.
I use to buy a HDH double tapered line and cut off the first 30' (I think that was where I cut it) and splice on a G or F size line. This was how the first weight forward tapers were built. The G or F size line was called the "running line" which was the first backing that I know about. It wasn't until the late 50's or early 60's that Cortland came out with Micron and that was when fishers found they could shorten the fly line and add backing. Micron has been the standard backing since that time. Now the new Gel Spun backing lines are finding favor especially with the salt water fishers.
Fishing with silk lines could be a pain. Silk lines became water logged after a few hours of fishing. One way around this was to use a double tapered line and after a few hours you would remove the fly line and turn it around. After a days fishing you had to remove the line, clean it and hang it to dry. Part of your fishing day was spent cleaning and dressing the silk line to keep it floating.
Why backing? Backing is not needed for a lot of fly fishing except to help fill the reel. I think the fishers that first needed backing originally was for Steelhead fishing where the fish were strong fighters and would take off down stream. In most cases you could not follow the fish as quickly as it could move down stream and you needed backing to keep from getting lined. As fly fishers discovered salt water and salmon and Alaska the need for backing became common place.
Thats my history lesson for the day.