Copying content from a web page is easy: Ctrl-C (Cmd-C on a Mac). Now, pasting this content in a text editor using the not-so-basic Ctrl-V command means “copy with all original formatting, including styles, rules, visible or invisible links, as much as you can, my dear text editor”.
Result is impossible to predict, as you're copying a lot of invisible things along with the text - and no matter which text editor you're using, you can hardly be aware of all the tricks used by its developers in order to make this magical operation possible.
One thing is sure: if you paste web content as-is within your text editor (Google Docs, Libre Office, Word, etc.), you might end up having a document that looks like the original, but on which you'll have very little control:
No view of what's behind the lines
Loss of all editor's main functions, such as Paragraph Styles control
Just add Shift to your paste key combination when you insert content in your document :
Windows, Linux : Ctrl-Shift-V
Mac : Cmd-Shift-V
In Google Docs like in many modern applications, this results in a “Paste without formatting” command, which will copy the text, and only the text. Yes you'll loose the original style, but this is the only way to control what will really be inside your doc at the end, and the best way to truly enjoy all Google Docs formatting features.
A well-formatted document in Google Docs, like in most advanced text editors, allows you to change all your text paragraphs in just a few clicks, without affecting your titles, or subtitles styles, no matter how long your document is.
Things like changing line spacing in all your text paragraphs, or changing all your section titles to grey background should be doable in one shot, which can only be accomplished if you followed a few rules in formatting your document, and is nearly impossible with content pasted directly with formatting from the web.
You may just don't care about all this - especially if all you need is a quick copy of something. But I tend to assume that if you've been searching for a tool like Show, you'd like some reasonable control on your resulting document.
Don't try using Show in order to correct a messy formatting coming from a web page copied in your google doc with Ctrl-V. Chances are, it will fail miserably, or at least, seem to fail.
i.e. you'll be sure there are tabs or carriage returns where there aren't, and Show won't show these.
i.e. content seems OK, but as soon as you run a Show function, doc appears messed up.
Do use Show if you know what's really in your document - and if parts of your doc have been copied from a web page, make sure you've pasted it without formatting.
Note that this advice is not particularly aimed at Show users, but at all Google Docs users, and more generally, true for all text editors.