Software as a service (SaaS) allows users to connect to and use cloud-based apps over the Internet. Common examples are email, calendaring and office tools (such as Microsoft Office 365).SaaS provides a complete software solution which you purchase on a pay-as-you-go basis from a cloud service provider. You rent the use of an app for your organisation and your users connect to it over the Internet, usually with a web browser. All of the underlying infrastructure, middleware, app software and app data are located in the service provider’s data center.
SaaS-Tenant enables a single instance of your application to serve thousands of tenants. SaaS multi tenant application saves users from arduous task of building robust, scalable cloud solution and let them focus on their business logic.
SaaS provider implies that you are moving from a packaged software model, a "product organization" to a "service organization" with hassle free management.
SaaS-Tenant can be deployed on public or private infrastructure, providing freedom from lock-in and tremendous increases in developer productivity, infrastructure utilization, and app management agility.
There’s just so many reasons for considering multi-instance over multi-tenancy, but let me give you one. An application that must run on-prem OR in the cloud. Build it once, my ignorant friend, build it once.