The problem it solves
Consistency across a portfolio is usually described as a visual nicety. It is not — it is a compounding cost.
When each product decides for itself what a button is, accessibility is done well in some places and forgotten in others, a pattern a customer learns in one product does not transfer to the next, and every team spends part of its time rebuilding what already exists.
How Qeet approaches it
The decisions are made once, tested once, and inherited. A change to how something works propagates outward instead of being re-argued in eight places.
The measure of whether it is working is not whether the products look alike. It is whether the next product costs less to build than the last one did.
Why it is a foundation
Along with identity, this is one of only two things the entire portfolio depends on. Between them they answer "who is this person" and "what should this look like" — the two questions no product can avoid and none should answer alone.