An hour-glass shape of the system gives both ubiquity and variety.

Typically the more options you have at a given layer, the more challenging it is to evolve the ecosystem.

Conceptually, each option at layer n has to interoperate with every option at layer n - 1.

This leads to n^2 combinations and creates considerable coordination headwind.

If you design the system with a narrow waist–a layer that has only a single general purpose option–then you allow easy speciation above and below.

Layer n + 1 and layer n - 1 can have as many options as you want, as long as they can go through the singular option at layer n.

This diversity allows the top and bottom layers to fit whatever niche is required, giving ubiquity of coverage.

This pattern shows up often in successful systems.

The internet's IP infrastructure has this shape.

LLVM's architecture has this shape, too: a single Intermediate Representation (IR) with numerous frontends (programming languages) and backends (target architectures).

My friend Dimitri has remarked on the power of this shape, too.