echo versus return in a snippet

Why is it considered bad to use echo in a snippet?

What advantage does return have over echo?

Many thanks in advance.

For one thing, using print() or echo can have the output inserted in unexpected places in your page.

For another thing, except for the most trivial of snippets, a snippet should use a form of user-configurable templating for its output. Its output should be loaded into placeholders, then a defined template, a chunk, a file, or inline snippet property, should be used to replace the placeholder tags in the template with the appropriate values from the snippet output.

Basically you should treat a snippet or plugin as you would treat a function or a method in any well-designed code.

https://rtfm.modx.com/revolution/2.x/developing-in-modx/basic-development/snippets/how-to-write-a-good-snippet

echo would be used for testing in a snippet. the snippet must return true, this tells modx that the snippet run sucessfully. I would probibly advise using print_r() for testing anything in your snippet, but still include the return true and the end of the snippet.