Why does a hotel need a basement if there were no hotel rooms there?

Where the staff can get wild with friends or guests. It happens. I worked in a hotel, for what it's worth!

Storage,  mechanical systems, laundry, employee areas, or storm shelter. 

Storage, perhaps. Space to do laundry, keep the huge amounts of sheets / towels / other bedding etc, toiletries, cleaning products that a hotel needs.

In some areas the bedrock is unstable, so it is almost as cheep to make a basement as just build a sufficient base for a building .

Probably where the laundry facilities are and storage for office supplies, linens, extra kitchen items like dishes, room supplies like soap and shampoos.