Why i need a wp loop in single.php

this was something that killed me for years and I found the answer: No, you don't need to use the LOOP in single pages, you just call the_post() and you have all data needed

.... BUT ....

If you don't use the loop (while(have_posts())....) a hook "loop_end" is not called and if a plugin/process has any action on this hook, it won't work. So for safety reasons you should use the loop.

Also, people ask do I need to check for existence before the loop: if(have_posts())?

<?
if( have_posts() ):
    while( have_posts() ):
        the_post();
        .....
    endwhile;
endif
?>

No, you don't need to check

.... BUT ....

Checking allows you to include headers/titles before the loop and not having them if the loop is empty.

The WordPress Loop instantiates some functions like the_title(), the_content() and others.

In other words, your post is loaded in that loop, and the loop is gone through once if you are on a single post. Although it might be strange to have a loop, it is actually quite useful.

WordPress uses a template hierarchy, which is a way of choosing which template to load for a given post/page. In your single.php, the loop will run once. But if you do not have a single.php file, WordPress will use index.php instead for that same post.

For the sake of consistency, having a loop which works for any number of posts is helpful. Else, in you index.php, you would have needed a case for one post and another case for multiple posts and keeping a consistent templating method would be difficult.