If the Notes app on your iPhone is anything like mine, it's a mess. Even with the addition of folders, adding note after note can really clutter things up. You need a way to quickly identify the notes that are most important, and that's where pinning comes in.
When you have an idea you want to jot down or a quick list to make, the Notes app on your iPhone is a great place to do it. However, by default, Notes use a blank canvas, which doesn't match the physical lined notebooks we're used to. If you've always use unlined or gridless sheets of paper, a blank canvas is probably fine, but you can actually customize your digital stationary with lines or grids.
When you hand your unlocked iPhone to somebody so they can use the internet real quick, check out some cool photos, or do whatever, there's a possibility that they could snoop around where they don't belong. If you have some secret recipes, login credentials, intellectual property, or other sensitive information in your notes, you'll want to add some protection to them.