score:18
When you are flying internationally, your passport data is needed.
If you type it in upfront, you save time when checking in. Also, many airlines offer to upload a photo afterwards, and get it verified. This allows you to check in online for international travel, instead of having to wait until you are at the counter in the airport; and with that you can pick your seat a lot earlier (unless you prefer to pay for early seat picking). When you are at the counter, they may have only middle seats remaining...
Also, believe it or not, many people realize only in the airport that their passport is expired or invalid (unsigned). When you type the expiration date in, you have a chance to realize it and get it renewed.
The data you enter is not binding; only your name is. You can always change the passport data later.