It is not necessary to be home, but it is up to you! When you make a booking, you will be able to leave entry and gardening instructions. If the gardeners can get access to your property, there is no need for you to be home. Or, feel free to stick around during the booking, whichever you're most comfortable with.
Just make sure your gardeners can access your property by following your entry instructions. If they are unable to access your yard, you will still be charged the full cost of the booking.