For starters let’s dive into what is 5 really. It is the post’s ID. But what is ID in turn? It is value in the MySQL table row which identifies the specific post record.

Issues with using IDs

So first there are some conceptual problems with it. It’s not content. It’s not something user creates, manipulates, or (most of the time) aware of. WP is (by design) pretty sneaky about exposing IDs to the users and almost never does it, spawning quite a few plugins around just for that purpose.

As a product of it not being content (or value meaningful for human) it’s not self–documenting. There is no way whatsoever to infer what post 5 might have been from your snippet.

And finally, while ID uniquely identifies post in specific database, it’s much less certain to do so as soon as content starts moving around between databases and/or even different systems.

So what are better options?

I’ve observed two typical reasons to retrieve and make use of standalone post.

Working with specific content in site

The persistent content is typically pretty static and in most cases can be identified by title. For example in your snippet low ID as 5 often refers to something like “About” page.

For retrieval of such content get_page_by_title() is fitting. Despite name it accepts $post_type as third argument and would work for not just pages.

So using better identification (and producing self-documenting code while at it) your snippet turns into:

$about_page = get_page_by_title( 'About' );

Working with specific place in site

Another case is when the post is used as a place in site, rather than specific content. For example we would like to retrieve site’s FAQ, but not sure (or care) what it’s precisely named and other content details.

But if we know where is it in site’s structure we can use get_page_by_path() function, very similar to the previous:

$faq_page = get_page_by_path( 'about/faq' );

Other cases

In more complicated cases there aren’t always well–fitting functions. Sometimes it boils down to using get_posts() with its rich querying options, even if only for a single post.