Emails and letters commonly end with a closing phrase such as
...but I never understood the reason why such a thing would come up in the first place.
Maybe it's because letters were (a long time ago) longer and better written, possibly a glorified form of communication that expressed more than just the message they contained, and closing them in a way that showed good taste, politeness, etc was part of that glorification?
In modern usage they sound very weird to me, like
I've sent the PDF with the report, be sure to backup before the maintenance this time!
So, why it was invented, or under what sort of influence it became the norm?