Say you notice they come in the morning, put a small machine on the grass, hit a button, and leave and come back 4h later to pick up the machine and charge you - wouldn't you be at least curious to see how much this machine costs? Or if there was another company that would lease such machines for much less than what you paid per hour, wouldn't you possibly prefer to contract them, even if you may have to pick up and drop off the auto lawn mower yourself?

And conversely, say that the person or machine that you paid to mow the lawn instead drenches your yard in herbicide and then salts the earth. Or vice versa, it starts planting weed seeds, or brings a bunch of ticks in the grass etc. You would probably not be happy to let this process finish and then say "oh well, guess I won't be hiring them again!" and just move on.