A primary use case is what gets your user to join.

A secondary use case can become one that gets your user to stay.

There's a class of secondary use cases that have a self-hoisting quality.

They start off small but can grow to the point where their activation gradient is above the bar and later they can sometimes become the primary use case.

But when they start off, secondary use cases are fragile seedlings that will wither in the sun; they need to stay in the shade of a primary use case to give them the space to grow.