One can work from the couch or living room table in a pinch, but if you need privacy, or the contract mandates a certain level of confidentiality (like other people not being able to see the screen, or having to lock paperwork away) you will want to invest in a separate room for work.

Also, at least in Germany, not sure about Switzerland, you can only write off the rent, taxes-wise, for a separate office room with a door. You can't write off a desk in a room used for other purposes or a space in a hallway.

Not to mention if you have roommates or a spouse and kids.