It’s “TH” like in “the”.

Old english used it because many words were from norse and vikings brought runes as an alphabet with them.

It neither survived that time english decided that french is cool, nor that time press was invented only for latin alphabet.

You can notice remnants of history of this letter by noticing how many english words contain “th” in them.

There is also another similar trope, defined by “Any problem important to the plot that can be solved by 5min honest conversation between participating parties”