Also, from a practical perspective, most large organisations don't have a system where you can go to your manager and say 'please can I spend $X of my own money

Also, from a practical perspective, most large organisations don't have a system where you can go to your manager and say 'please can I spend $X of my own money on my monitor/chair etc' and then have them buy them for you. It might work in a start-up but the infrastructure for that isn't in place in most organisations - you could bring stuff in yourself I guess but it wouldn't be insured. Likewise, my (carefully adjusted!) chair regularly finds itself in some other part of the office when I'm not in, and if I'd paid for it myself, that would be mighty annoying.