If you have an enforceable contract, the client is obligated to pay for the services received.

As a business owner, I would be very concerned if a SAAS was demanding upfront payment for 2 years.

A multi-year contract helps you ensure steady revenue for your business. So long as it can be properly enforced.

There can be different payment models: monthly, yearly, full contract upfront, etc... And you can discount each of the terms appropriately. If I were a customer I would want some flexibility in how often and what I pay for.

But just like everything else, no payment / no service.

You tie in the customer.