I'm curious about the history and purpose of the top-leftmost key on UK keyboards.
The primary symbol is the backtick ` which I know is useful in some technical contexts (e.g. Perl programming and Linux shell scripts), though most everyday users would never need it. Shifted, the key produces ¬, which I recognise as a NOT operator from formal logic, but again have almost never seen in use: the programming languages I have encountered use ! or ~ for negation.
With AltGr, the key produces ¦, which seems particularly obscure. Again the pipe is useful in some programming situations, but that's a normal pipe | and I have never seen this broken pipe used anywhere.
So why, historically, is there a key for these rather obscure symbols? I would have thought that there are many symbols that would be more helpful to most everyday users, such as the copyright sign, or the temperature degree symbol.