In *theory*, the rulebook is black and white and covers every case. If you c*ck up, the union will not be any help. If you just turn up every day and do your job properly you will never be under threat of being sacked. All in theory is good and rosy. But...
In *practice*, every day we are expected to do things that are borderline cases, or are not considered in the rulebook, or we have been given contradictory instructions on, and so on. If a driver makes what turns out to be the wrong decision and their company want to kick them out, the worth of the union to the individual becomes obvious. In practice there are a lot of areas that are grey, and the battle for your job will be won by whoever can afford the best lawyer. Your reps will know of similar situations that others have been in, will be able to highlight training issues across the company, will know what mitigating issues lie in your favour, etc.
It is the same in situations with medical or social issues. You may be fine now, able to turn up to work every day without fail, but life randomly throws up surprises. Relationship and childcare or geriatric parent problems outside of work causing an issue and company being difficult? Long term moderate health issues appearing out of the blue, like depression, IBS, PTSD, long COVID, etc? Difficulty recovering from a fatality? Cancer? Heart disease? Your company will never tell you what the best options available to *you* are, as it is in their interest to only tell you the one option that is best for *their* interests. However your union reps will know what is available and what reasonable considerations the company can and should make to help you work through it.
Before I joined the railway I spent 20 years in the software industry, where there is no unionisation. If my ability to drive were lost and I had to make another career change, my decision as to what to do next would now be strongly influenced by whether there would be collective representation of my interests in my new career. I am not currently looking to move between TOCs, but if I were, lack of union recognition would be a huge red flag against a potential move.