You often (depending on region and property) have to pay some kind of ground rent and/or non-trivial tax that you can transfer to the occupant. Also they can pay for maintenance and also point out big problems (water leaks - yes they do happen and be very unwelcome even if you have insurance). Overall, the net gain is low but that's better than 3 or 4 years of a minor accumulating net loss. Also the management fee is say 10% to keep your place in shape, so actually quite cheap in reality if your own time is in any way expensive.

[Edit: multiply the costs if you have a portfolio of properties and you then realise that you can easily have a dent in your cash flow while your property is accumulating in value]