As a writer, the em-dash is one of the most common characters I use that isn’t available easily on keyboards. Most word-processor applications will automatically convert two consecutive hyphens “- -” (no space between) into an em-dash, but there are lots of other scenarios where you might want to use one.
The Punctuation Guide has a good summary here.
Incredibly basic call out — you use a hyphen within joined words/names John Smith-Jones or mother-in-law. Em-dashes are physically longer (horizontally) and are used to join ideas / sentence parts. From the Punctuation Guide:
The em dash is perhaps the most versatile punctuation mark. Depending on the context, the em dash can take the place of commas, parentheses, or colons—in each case to slightly different effect.
iOS doesn’t provide a function for inserting “special characters” like the em-dash. If the software you are using doesn’t automatically convert double hyphens to em-dash, then you have three options:
Option 1 is perfect, unless you do a lot of typing on your ipad using the keyboard cover (like I do).
Option 2 is great — I didn't know about that keyboard shortcut originally.
Option 3 sucks. Don’t do that.
Option 4 is easy to set up, and then works like a charm if you can't remember the keyboard shortcut.