With the purchase of a property you become a member of a community of owners, which is subject to many legal regulations. If you do not use the property yourself, the special property management is responsible for looking after your rented apartment.
The distinction between you as the landlord of a property and a member of a community of owners requires special knowledge and, of course, time. This is where an administration can significantly relieve you.
In short, all matters that belong to the proper management and care of special property together with the associated special rights of use.