Some people - many people - benefit from Protection.
By this, I mean that their employer then won't do any of the following;
Insist they work in a loud, crowded office where there is no chance of being able to concentrate because there are five separate conversations going on, constant visitors to other staff, flickering fluorescent lights and no natural daylight, a huge amount of clutter, a radio going all the time, stuff on the walls and visual noise shrieking at them all the time (including situations where they've previously been working in a more or less ideal environment but somebody else decides that everybody should be in the same area and it's not fair that one person gets a 'privilege' to have their own office).
Ban people from getting up and moving around. 'The only time people leave their desks is during their designated break. If you want to get up, you agree in advance with the manager that the rota will be changed and you can take your lunchbreak at 11.30am from then on.'
Expect them to have lunch at their desks or to miss lunch/come back when summoned. 'Oh no, there's work to do, if you go over your time for lunch, you don't get to take it later'/'Where are you? What do you mean on lunch? You're needed back immediately, there's work to do'.
Impose changes with zero notice 'Oh, we've decided you're working over there from now on, next to the person who has constant visitors all day/the fridge that buzzes and will have people going to it and complaining to you that there's no milk like it's your problem, you've got fifteen minutes to move and then we'll expect you to be running at full capacity', 'No, from now we've completely changed a process and you now have to go in and out of three separate apps and everything has to be manually input because copy & paste won't work but we haven't written this new process down or thought that this way makes another aspect impossible'
Insist that everything is done in a rigid way that doesn't take into account a preference for visuals or simple logic - such as a phone list that is ordered by Mr A - Z, then Miss A - Z, Mrs A - Z, Ms A - Z and is a word list converted to PDF, so can't be edited and done properly colour coded/reorganised into a logical system.
'Why can't you answer right now? Don't you know? Why haven't you done it? Why haven't you said you've done it? I've emailed you fifteen times in the last hour requiring other things, why haven't you done the first thing yet?'
'Well, I'm sorry that you feel that way about the new office setup, but we are not going to give anybody special treatment, so we'll go forward with you reporting to x person with a written account of your activities by 9.05 every morning and an hourly written update and then a written account of what has been finished, what is outstanding, what you are doing to resolve them and how many minutes it will take for you to complete them so we can see where you're going wrong by 4.10pm every day'.
Those things may well affect everybody, but the protection that a diagnosis gives, it's less likely to be happen because a) they know it's a bad idea, b) they might listen if somebody says it won't be helpful due to their ADHD whereas an alternative works because or c) they would be at risk of getting into legal hot water due to deliberately doing those things to make it unbearable for the person to remain - so constructive dismissal - or to set them up for Capability Procedures to manage them out.