Most timeshare owners must go into debt to purchase their timeshare from the developer. They go into debt so willingly because they are often told they can rent out their unit and actually make some extra money. The problem is that timeshare owners generally have no idea how to effectively rent out their unit. The resort is more than happy with this situation as they usually provide a service to rent out the unit for the owner -- for a FEE. This fee is typically around 35% of the rental price. And they don't guarantee a rental. I fit doesn't rent, you're out of luck. This is definitely not a guarantee.
Every timeshare unit also comes with yearly maintenance fees. These fees cover up keep and taxes at the home resort and range from $500-$1000 per unit per year. Often an aggressively-priced timeshare rental doesn't even cover these fees.