Simplest reason is because I wanted it!
My road bike is a necessary evil so I can enjoy longer rides than are feasibly with my favourite bike.
My favourite bike, I bought in the hope it would be low-maintenance as it has hub gears and drum brakes. I have wrecked one hub gear, but on the whole I think I've spent a lot less time and money maintaining it than I would a derailleur geared v-braked bike so I'm happy I made the right choice (and it's mainly about aesthetic anyway).
My folder I bought simply so I could do some rides starting further afield. It has not disappointed, but being 2nd hand and having been kept on a boat has needed a fair bit of maintenance. I've since bought a car rack so I arguably don't need the folder any more but it comes in handy sometimes as a spare.
I'm always thinking about n+1, but to be honest I don't really want another pair of tyres in my garage to keep pumped up so have managed to avoid the temptation for the last few months. If I do get another it will be as a winter bike so I don't have to swap to winter tyres and back again every year.