Projects are the main entry point for all Serialized APIs. Your API keys are connected to a single project and your data is stored in a single project.
There are many scenarios where using multiple projects can be useful:
You can use different projects for separating data in different development environments (development, test, production).
You can provide teams with their separate projects where the data for their services can live in an isolated environment from each other.
You may have different internal development projects that should not be dependent on each other or interfere with each other, but you may still want to keep them under the same Account to collect your billing in an aggregated way.