An Owner is the legal entity which owns one or more of your properties. An Owner may, infact, be more than one person grouped together by a partnership or company.
It is not uncommon for a person to own properties through more than one legal entity and, for taxation purposes, the records need to be kept separately.
Alternatively, if you helped a friend or client to manage properties you would need to be able to identify which properties, receipts and payments were relevant to them. Creating an Owner for your friend and attaching the properties to that new Owner would let you do this.
For this reason PC Property Manager requires you to create atleast one Owner in your system.