To put it simply, they are the middle person between host and short-term rental platform. What they offer you is completely hassle-free management of your property: they do the price setting, the advertizing, and provide the services - you just sit back and enjoy the extra income. Additionally, they take a lot of care in selecting reliable guests and personally welcoming them and seeing them off, as they are fully aware of how precious your property is to you. Many of their customers previously used their apartments for long-term letting, and dared to switch to the more profitable tourist market only because of their expert guest selection process.
In addition to taking over the full management of your property on Airbnb, including all services, they also offer standalone services. If you prefer doing some things yourself, they can of course still help you with the rest - you can pick exactly what you want!