The question has been answered quite thoroughly. But since the question quoted me directly (from my answer to the overnight detune question), I feel compelled to weigh in.
Part of the reason I have ten acoustic guitars is purely nostalgic. My first acoustic was a present from my wife from before we were married so it has sentimental value. My next acoustic was a Christmas present from my Brother - so it has sentimental value as well. Over the years I have come across guitars, new or used, that I wanted to buy either because I liked the way they felt or the way they looked or the way they sounded or a combination. So I bought them. I have sold a few guitars that I had no particular attachment to. Most of the ones I sold, I had acquired because I got a "good deal" on buying them.
Of the guitars I have kept that have no particular sentimental value - they all serve different purposes. I have three acoustic guitars that I use primarily for performing. These all have electronics so I can plug them in. I rarely play these unless I am on stage. I have three because I like to have a backup guitar at all times in case I break a string and I leave them in different tunings for different songs.
Some of the guitars I play at home live on guitar stands in different rooms of my home. I have a couple in different tunings in my office/recording studio and I have my favorite couch guitar sitting on a stand beside the couch in the living room.
One very practical reason for playing different guitars is that if I played the same guitar every time every day, I would wear out the frets too fast. So I like to rotate guitars and play different guitars on different days. I do have my favorite guitars, but I try to play the others also -to preserve my favorite guitars.
I have several more guitars on my wish list, so I hope to add two or three more acoustics to my collection soon. There is not any correct answer to this question. I have several guitar playing friends who have collections larger than mine who each have their own reasons for keeping the guitars they keep.
Like my guitars, there is a unique story behind every single one of them. People collect different things (coins, stamps, model airplanes) as a hobby. I consider collecting guitars a hobby of sorts, but unlike coins I can't spend, or stamps I can't mail, I can play the guitars in my collection and thereby derive enjoyment beyond just the intrinsic value as a part of a collection.
These are some of the reasons my acoustic guitar collection is at ten and growing. Thanks for asking.