$ for file in ./tmp/*.txt; do printf '%s\n' "$file";  cat "$file"; done

-or-

$ find ./tmp -maxdepth 1 -name "*.txt" -print -exec cat "{}" \;

There is no difference from a user point of view. These commands do the same thing.

Technically the difference is in what program opens the file: the cat program or the shell that runs it. Redirections are set up by the shell, before it runs a command.

(So in some other commands--that is, not the command shown in the question--there may be a difference. In particular, if you can't access file.txt but the root user can, then sudo cat file.txt works but sudo cat < file.txt does not.)

You can use either one that is convenient in your case.

There are almost always many ways to get the same result.

cat accepts a file from arguments or stdin if there are no arguments.

See man cat:

SYNOPSIS
       cat [OPTION]... [FILE]...

DESCRIPTION
       Concatenate FILE(s) to standard output.

       With no FILE, or when FILE is -, read standard input.

I used grep for something similar:

grep "" *.txt

It does not give you a 'header', but prefixes every line with the filename.

$ for file in ./tmp/*.txt; do printf '%s\n' "$file";  cat "$file"; done

-or-

$ find ./tmp -maxdepth 1 -name "*.txt" -print -exec cat "{}" \;