why a lot of tags must be within the loop?

get the ID: “Retrieve the numeric ID of the current post. This tag must be within The Loop.” i know. if you get a lot of posts, this tag must be within that loop that it will change along with every post.
But, i put it without the loop in single.php, of course above “while(have_post)…..”, it ran well. i checked. should i use tag put without the loop if i have only a post like single.php file?