Sticks like the proverbial and finds it’s way into small gaps and seals them nicely.
Used it plenty of times over the years. Most recently on my current workshop which is elderly wriggly tin. It’ll buy a bit more time till I replace it.
You can also paint it on walls if you get ingress. I’ve done a strip around the bottom where there’s no damp proof course.
I had to recoat some Onduline sheets - used Bitimous Paint (same stuff as Black Jack) but available a lot more cost effectively.
Got a 25L drum of their own brand from these guys https://njldirectlimited.co.uk/
They do some different ones - I think some have fibres in to repair leaks.