Answers: Basically because if you sell a house which consequently causes the foreign owner a problem, he/she could sue you.
For example, if the new owner be injured by falling through rotten floorboards or if a damaged chimney fell on him or if a wall collapsed on him or if he be 'blown up' through faulty electrics ... Need I go on? You would be liable. Hopefully, surveyors' reports should take care of such things - but you never know.
It would be of no use maxim that you did not know about a extraordinary fault. In the worst bag, if a person be killed, you could resourcefully be sued for astronomical amounts - and much more than the value of the house!
Actually, even whilst you are still the owner, you enjoy a duty of care. If a guest were hurt by something wrong near the house or garden, you could well be within the same position, so that indemnity insurance is other worthwhile.