They are a legal requirement if you're going to be marketing your property now. (Not if its a private sale with no agent involved)
They hold an EPC (energy performance certificate) similar to the one you would get on a fridge/ washing machine/ light bulb.
They are a complete waste of time, the idea was initially that the vendor had to get a survey done on the property and this was to be included in the HIP. this part of the HIP was scrapped, so now all it holds are some searches, drainage/land reg, and the EPC.
A complete waste of time and money. But its a the law !