The title says it all, really. All punctuation (periods, commas, semicolons, quotation marks, and so on) functions like road signs, but for readers instead of drivers. The cleanest metaphor is a period; it’s a stop sign. It signals the end of a thought. Commas signal a pause (not necessarily a breath!). Semicolons signal a connection of some kind. Parentheses signal additional information that isn’t required but is helpful. And so on.
In the same way that traffic signs keep cars from running off the road, punctuation marks keep the reader from ending up in the word weeds. At the same time, they help keep your thoughts in line, which you’ll see in action as you’re writing, I’ll wager. You type, then back up and add or remove a mark, then type more. You’re placing, adding, and removing road signs for your readers. A colon might mean “A list of items is coming up.” An em dash can signal “There’s a sharp change in direction here.” Quotation marks mean “Someone is speaking” or “This material is taken directly from a source.”
Is someone pausing to think in the middle of speaking? An ellipsis (three spaced periods with a space before and after, in Chicago style) lets the reader know it’s happening.
“I’m hungry for Japanese tonight. Let’s go to . . . How does Shiroi Hana sound to you?”
Are they changing thoughts midsentence? An em dash shows readers that.
“Sounds great! We can take my—Where are my keys?”
[There are other ways to indicate this by using beats. However, this post isn’t about that. It’s good to know more than one way to lead your readers in the right direction.]
And, as always, remember: I write about American English. If you’re using a different English, the rules and guidelines may well be different.