Imagine we're working together and I have some text document I want you to work on and update. If I send you the text by email, I send a
copy of it (the original text is still on my computer). If you modify that copy and keep it for you, I won't ever know what you did. I just used a function
func (c Coworker) SendForUpdates(d Document) {
...
}
That wouldn't make sense. You worked hard and I don't even know what you did. So, what I would expect you to do is, once you made updates on the copy, to send me back that copy by email. That would be akin to
func (c Coworker) SendForUpdates(d Document) Document {
...
return d
}
I sent you a copy, and you returned another updated copy. That is "pass-by-value", the default, no-pointer style.
Now, let's say I think those emails back and forth and boring. Rather than sending you a copy of the text each time, I could rather use Google Docs, and send you the link to that document. Its URL, rather than a copy of its content. Now, you can just go to that URL and do the updates on the document. You don't have to send me back the document: you're working on it, not on a copy of it! Well, that URL is a reference to the document rather than the document itself, or, if you prefer, a pointer to it. So, now, the function would be
func (c Coworker) SendForUpdates(d *Document) {
...
}
And we're done, no more back-and-forth dance now! That is "pass-by-reference".
You don't only use "pass-by-reference" just to be able to check updates on the document sent, by the way. If I want to send you some text just for your information and I don't expect any kind of update, I'll use pass-by-value (the very first function). But what if I want to send you a 3 GB video? I can't send that through e-mail! Sending a copy would be totally inefficient. Once again, I'll send you a pointer, an URL to download the video:
func (c Coworker) InformText(d Document) // d is small: pass-by-value
func (c Coworker) InformBigVideo(v *Video) // videos are huge: pass-by-reference
Why not use pointers everywhere by default, they seem easier, right? That's basically what java and python do. Well, they can be tricky too. I gave you the URL to the link and you could work on it. Once you're done, I don't want you to modify the document anymore. I want to send it to our boss. But, how could I know you didn't keep the URL somewhere in your bookmarks? How do I know, of all the coworkers I sent the URL to, one of them doesn't keep on updating that document even when I don't want to anymore? With copies, I'm safe, do whatever the hell you want with your copy, I don't care anymore. But a reference to the original document? That can be dangerous.