When you build a new property or purchase an existing structure, sure, the goal is to secure tenants. However, there is no guarantee that you will be successful. In fact, as a property investor, things are always changing. All your units could be occupied at the start of the year, and then empty by years end. Insurance protection can be especially helpful during these times.
Whether you own a single-family home or a building with multi-family units, you don't have rental income when you don't have a tenant.