Factless fact table

What is a factless fact table?

When a fact table does not have any fact is called factless fact table. It has only foreign keys of dimension tables.
How will it be useful?
It can be used to view a report of whether any event has occurred or not instead of any data aggregation.

When you generate a report, we can search whether a particular conference room is available or not for a specific day.
The report will give the information of particular conference room is available for the selected day or not.