Metasyntactic variable

It is necessary to use them because they simplify the understanding of the examples.
Everything that is not important for the compilator, are not keywords, is better to name it this way.
There is the wikipedia article on this topic.

List of words

foobar, foo, bar, baz, tata,   
titi, tutu fuga, qux, quux,   
corge, grault, garply, waldo,  
fred, plugh, xyzzy, thud;

Although other short words also are suitable for example- fee, boo, loo, koo. Why not?
But it’s still preferable to add tags <username>, <host> ets.