Survival tins are a pet hate of mine.
When I was starting out and knew next to nothing, I was quite intrigued by the idea of a tin and like many before have posted, had a go at making my own up and enjoyed the process of doing so. Then as my knowledge, skills and experience grew; I found myself thinking such tins where actually a waste of time, for anything other than them being an exercise in identifying useful, minimal kit.
You can tie yourself in knots with 'What if?' thinking, but in my experience, in Britain, even in the remotest areas, you are rarely more than a days walk, hobble or crawl from help. And, if you are injured to an extent you can not even crawl to that help, then you are not likely to be in any fit state to catch or trap, let alone kill, prepare and cook anything either. A space blanket and Poundland survival whistle would serve you better
Some of the items carried in a standard tin, can be used for none survival purposes, e.g. fishing line as thread for repairs, but a lot of folks anticipate as many scenarios they may encounter as possible and choose the kit they carry to cope with such eventualities.
Proponents of the tin usually pip up at this point and argue 'What if you loose your pack and all the gear in it?' which is a very good point. If for some reason you became separated from your pack, the contents of a tin may prove useful, but only if you carried the tin on your person and not in your pack with the rest of your gear, as a lot of folk do, making the tin both redundant and unnecessary weight.
If I were a fighter pilot of special forces operative, flying over or infiltrating enemy territory, then I can imagine several scenarios where a survival tin carried on my person could save my life. But as I am neither, I am struggling to envisage a single scenario where a tin would be remotely useful, while hill walking and wild camping in Britain.
Survival tins are, in my opinion, useful for teaching purposes and making one up can be an entertaining way to focus the mind, but beyond that, the bushcraft equivalent of t:ts on a bull.