I think they serve a purpose, if say, someone inherits a property hundreds of miles away and just wants rid of it, but without all the implications of having to keep it maintained and secure while on the market. They also offer completion within 28 days and as I understand it pay all the costs. The drawback is, of course, for an instant sale, the price offered is going to be about 80% of the average value of similar houses in that area. These Companies know you are desperate to sell, or you can’t move forward otherwise, or you’re happy to take a lower figure for the convenience of having the cash in the bank.
I neither condemn nor condone them. Some might say they offer a service, while others say they take advantage of someone’s unfortunate circumstances.