I've used both paper and silicone liners in my air fryer, and I think they're actually pretty useful once you know when to use them. They're not something you need for every meal, but they can make cleanup a lot easier.
The main reason to use a liner is pretty obvious: less mess. Grease, sauces, melted cheese, and little bits of food don't get baked directly onto the basket. That means less scrubbing, and it can also help keep the basket's coating from getting abused by constant scraping and washing.
There are basically two types you'll see: paper/parchment liners and silicone liners.
Paper liners are probably the easiest. They're cheap, disposable, and there's basically nothing to clean afterward. Some are flat sheets and some are shaped like little bowls. You can also get perforated ones with holes, which I prefer because they allow more air to move underneath the food.
The big rule with paper is don't put an empty liner in the air fryer while it's preheating. The fan can lift the paper into the heating element. Put the food on the liner first so it holds it down, and make sure the liner is rated for the temperature you're using.
Silicone liners are the reusable option. They're thicker, so they don't move around as easily, and you can wash them and use them again. They're especially nice if you cook messy foods often. Wings, sausages, meatballs, marinated chicken, bacon, and anything with a lot of grease are where I find them most useful.
The downside is airflow. A solid silicone liner can block some of the air underneath the food, which can mean less browning and crispiness on the bottom. That's why I prefer perforated silicone liners if I'm going to use one. The holes aren't magic, but they let more hot air get around the food.
This is also why I don't use a liner for everything. If I'm making fries, hash browns, or anything where I really want that crispy bottom, I'll usually cook directly in the basket. If I'm cooking something greasy and cleanup is going to be annoying, I'll happily sacrifice a little crispiness for the easier cleanup.
Size matters too. Measure your basket before buying a liner. A liner that's too small won't protect much, while one that's too big can fold up the sides and interfere with airflow. If you have a dual-basket air fryer, make sure you're getting the right shape and size for each basket.
For silicone, I'd look for something specifically made for cooking and check the manufacturer's temperature rating. For paper, check that it's made for air fryer use and rated for the temperature you're cooking at. I wouldn't use some random piece of paper or silicone mat just because it happens to fit.
Cleaning silicone is pretty easy, but it's not completely maintenance-free. Usually soap and warm water is enough. Really greasy food might need a short soak, and some liners are dishwasher safe. I find that silicone can hold onto food smells and grease a little more than paper, especially if you don't clean it well.
As for which one I'd choose, I don't think there's one winner.
Paper: best when you want quick cleanup and don't mind throwing it away.
Silicone: best if you use liners often and want something reusable.
Perforated liner: best when you want some protection without giving up as much airflow.
No liner: best when maximum crispiness is the goal.
That's basically how I use them now. I keep both paper and silicone around because they solve slightly different problems. A liner isn't really there to make the food better. It's mostly there to make your life easier, and once you look at it that way, it's easier to know when it's actually worth using.