@Lorna170
The >how< is very easy.
The >why< is more complicated.
First, it's important to know that at least in my case, the main objective of it is to scare away risky guests.
We have a countryside property in a very popular tourist destination. I receive dubious enquiries and requests very often. It's at least once a week, sometimes daily.
If it's just an enquiry, there's not much need to discuss it. There's no obligation to accept or decline. Adios.
But if it's a request, I have to clarify their motives and decide how to proceed. There is a penalty for declining requests. Especially if you're forced to do it often. I ask a lot of questions.
So, if in the vetting process, I discover that it's a group of 18-25 year old males, I indicate that since they statistically represent the highest risk group, I will require a 1000€ deposit. Insurance companies charge triple for high risk groups. I demand a security deposit.
Usually this scares them away, which is just fine with me, because I'm not handing my house over to a group of 18 year olds without any security. If I don't require a deposit, then they have no motivation to avoid damage nor leave the house in any kind of order or cleanliness. None at all. There is no penalty. Go ahead, trash the place.
But 1000 bucks is a very effective carrot. In the handful of cases where they've agreed to it, they've left the house in amazingly good condition.
It works extremely well. I couldn't care less if Airbnb likes it or not. And so far, nobody's complained. But I'm not handing my house over to a bunch of kids without some security. It's my house, not Airbnb's.