It might happens that you're going to develop a website which includes a blog. It could be interesting to split the APIs into two different projets. However, it can be annoyed to deploy on two different servers for such a little API. That's why the multi-tenancy is interesting!
The multi-tenancy (or multi-projects) changes the architecture of an application to allow multiple projects in a single Strapi application. Each project is fully independent, you can use different databases, configurations, plugins, etc.