Ttyjukl

It seems that the purpose of cat is to concatenate several files. However, many people still use cat instead of less (or a similar program like more) to display a file. See, for example, the GNU m4 manual.

Man page: less

-F or --quit-if-one-screen

    Causes less to automatically exit if the entire file can be displayed on the first screen.

-X or --no-init

    Disables sending the termcap initialization and deinitialization strings to the terminal. This is sometimes desirable if the deinitialization string does something unnecessary, like clearing the screen.

Nowadays, is it a good practice to use cat to display or view a file? Why use cat to view a file?

This makes me think to Useless Use Of Cat.

Note: This question is not about the differences between less and more. Moreover, it concerns the visualization of a file created earlier.

I'm going to assume that the "many people" in the question refers to people writing tutorials, manuals, or answers on web-sites such as this one.

When writing terminal commands in a text document, the cat command is commonly used to show the contents of a file.

An example of this:

$ cat script.sh

#!/bin/sh

echo 'hello'

$ chmod +x script.sh

$ ./script.sh

hello

Here, I show that I have a file called script.sh, what its contents is, that I'm making it executable and that I'm running it and what the result of that is.

Using cat in this example is just a way of "showing all one's cards", i.e. to explicitly display all the prerequisites for an example (and doing it as part of a textual representation of a terminal session).

less and other screen based pagers, depending on how they are used, would not necessarily give that output in the terminal. So if I wrote

$ less script.sh

#!/bin/sh

echo 'hello'

and a user tried it by themselves, they may wonder why the text of the script appears different in their terminal and then disappears from the terminal once they closed the less pager (if that's the way they've configured the pager), or whether their less is different from the less used in the answer (or tutorial or whatever it may be), or if they're doing something else wrong. Allowing for the possibility of this train of thought is counterproductive and disruptive for the user.

Using cat when showing an example in the terminal as text is good as it gives a fairly easy way of reproducing the exact same results as in the given text. For larger files, it may be better to show the file separately, and then concentrate on how that file is used when writing the terminal command as text.

If you prefer to use less, more, most, view, sublime, or some other pager or program to view files, that's totally fine. Go ahead and do that. But if you want to provide a reproducible text describing some workflow in the terminal, you would have to also give the user a warning that the output may differ between what they read and what they see in their own terminal, depending on what pager is used and how it's configured.

less is a non-standard GNU utility.

cat is POSIX:

     The Open Group Base Specifications Issue 7, 2018 edition

      IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)

           Copyright © 2001-2018 IEEE and The Open Group

NAME

cat - concatenate and print files

SYNOPSIS

cat [-u] [file...]

...

cat is more likely to exist and to have consistent behavior.

IMHO, I guess it's an "old / bad" habit. Myself, I always use cat because I do not think about less that is more suitable.
If you try to display the contents of a binary file, less asks if you agree to display it anyway and automatically paginates a text when it is long.
Cat is short to write and is easy to use for very short files that do not require paging.
Nevertheless, it is indeed intended for concatenation. In this case, used to display text, it performs a concatenation on the standard output of the file given in argument 1 and the standard input which is the default of argument 2.