Unix's first job in life was as a documentation processing system. It was made to be very good at dealing with text. All of the tools which process text expect every line to end in a newline. The last line is not exempt from this. All classic Unix text editors will automatically append a trailing newline to any text file you create with them. Some modern tools may be _tolerant_ of omitted trailing newlines, but you shouldn't rely on that. A text file should always have a newline as its last character. Otherwise, it's not _really_ a text file.

POSIX defines it more succinctly than I do: A text file contains one or more "lines" and every "line" is terminated by a "newline."

https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1...

https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1...

Some old tools had bugs where they'd read a line (up to the new line) and then process it, so if the last line didn't end with a new line they'd never do the processing. So a manual workaround for bugs became the convention.

so that you can cat(1) multiple files at once, and their bookends don't get glued.