The Spent Nights Report is an overview of how many guests stayed at the hotel on a specific date. This may be important for statistical purposes.
This report is often used for reporting city tax amounts to the municipalities.
You can filter the report by segments, to see how many guests stayed at the hotel from each segment. This will help if you for example only need to pay city tax for leisure guests, so you can filter out the leisure guests.
You can also filter out guests based on their age, again this is important for city tax calculations, as you often have to pay city tax only for a certain age group.