I see a lot of the larger bags are large amounts of putters and mids, and I can understand that, because I personally believe the short game is the most important.
Basically, I see a need to be able to throw a disc left to right, right to left, and straight. This can be accomplished with one disc, but using understable and overstable discs make these tasks much easier and more consistent. So then you have distance considerations. There is a sizable distance difference between my roc, my teebird, and my boss, so using different speed discs makes judging distance easier and more consistent. Then the touch/landing. My putter and my Roc tend to glide in easier, less skip, and in tall grass can stop on a dime. Using a driver can cause a great skip and roll, so using all the discs in the short game can get you into hard to reach positions if you know how the discs react when hitting the ground, another great need for having various type of discs. And rollers, I find like throwing, understable will turn up straight, then go right (RHBH) much easier than overstable discs, so depending on the distance, and which way you want the disc to turn, you need to roll all of your discs in your bag to know which does what easier. Forehand/Backhand shots need to be practiced likewise.
All this ramble to say, I have met people who have certain roller discs, certain forehand discs, certain backhand discs, etc. And the discs they have all overlap, and I am always