The advantages of having someone you don't know house sit for you rather than asking a friend or family member to help you out are numerous.
Many an informal agreement between friends or family members has led to the damage or destruction of previously good relationships. If you do require a house sitter it makes much more sense to engage the services of a stranger rather than jeopardize a close relationship.
Essentially, with a house sitter, you can conduct the whole arrangement in a very business-like fashion, asking them for all the usual reassurances that they're trustworthy and responsible. Both you and your house sitter will undoubtedly be keen to go through a house sitting agreement together to spell out exactly what your obligations and expectations are of each other. While the relationship is still one based on trust (as with a friend or family member) you should feel no apprehension in asking your house sitter for a security deposit.
With a friend or family member you'll most likely feel that you need to simply 'go on trust' in arranging the terms of the house sitting agreement which often means not discussing the terms of the agreement at all. One home owner's kind-spirited offer to his friend and house sitter to 'help herself' to his wine collection ended in dismay when he returned home to discover she'd helped herself to two dozen of his prized reds. With a house sitter, he could have guiltlessly locked his wine in the cellar, adding a caveat in the house sitting agreement that it was not to be touched.
Crucial to the distinction between engaging a house sitter online and a friend or family member is one of personal motivation. If your friend or family member is, in essence, doing you a favour rather than agreeing to house sit for their own reasons there's every chance they'll let you down. That's human nature. Here at MindMyHouse we're trying to work with human nature in providing a forum for people to meet to satisfy their own needs while meeting another's house sitting requirements at the same time.