I have MTB's and Road bikes. Road bikes----- A Boreas Ignis that is a 15lbs- Head between the knees out and out race geometry bike that is great to ride-for up to around 60 miles. Fantastic bike and works. However I also have a Giant TCR-C that I have set up for longer rides and hills. Each has it's own use and either bike "Could" do either type of ride. However they are both high end bikes and as such can walk very easily. I needed a runabout beater bike for local shopping and was thinking about a cheap CX bike to give yet another bike for a different use. Thought about it and I have the MTB's for the use that would be given so got a Pinarello instead. Only the cheap FP UNO but it has proved its worth. To save using the TCR or Boreas on the wet rides- or local short rides-I use the Pinnie. Still didn't have the beater bike but one came my way in a 2 year old Hybrid with Nexus 8 spd gearing

Each bike I have is used and used in the fashion that it was bought for. Only one I don't class as a bike yet is an old "Dawes Galaxy" that needs rebuilding so it sits up in the rafters of the shed.

So your question about why should we have multiple bikes can be answered because they are required. The real question should be "Where Can I buy a Bigger Bike Shed"?