cat takes a file as an argument and prints the file to stdout.
cat can also print to stdout from stdin like this:
> echo abc | cat - abc
Which is the equivalent to:
> echo abc | cat abc
But when you need to concatenate a line to the beginning of a file and then process that result it comes in handy:
> echo abc | cat - 123.txt abc 123 > echo abc | cat - 123.txt | grep abc abc
cat man page (here) has a history section and mentions that:
Dennis Ritchie designed and wrote the first man page. It appears to have been cat(1).
cat was the first man page, interesting! (The ubuntu man page for cat
does not mention this)