They are supposed to record speech faithfully, signify book titles and point attention to nicknames, among other things. A hyphen is well-used in our language. It creates economy by joining modifiers that belong together. Ellipses warn us that something is missing . . . . Notice that no apostrophe is needed.
Some students have problems discerning the difference between plurals and possessives.
These examples should clear up the problem. Its vs. it's The price of a gallon of gas reached its peak in the 1970s. You're Your pants zipper is unzipped. You're going to be embarrassed if you don't zip it up soon. They're going to stay at day care center until Joe picks them up.