I mean, it's fundamentally just a graph, but it's a way of correctly and efficiently computing changes in massive, dynamic graphs. Let's imagine you have a diamond shaped subgraph that fans out to hundreds of intermediary nodes before collapsing down again via and paths with different "lengths". And what if some of those paths have e.g. min(A, B) where the max side is the only one changing?

A naive observer approach will 1) compute that potentially exponential blow-up very inefficiently and 2) probably have "concurrency" issues. This library will be close to optimal and correct, even if you start dynamically changing the graph structure.

But yes, you can achieve the same thing with observers and other kinds of approaches. Most of them just a lot harder to get right while avoiding performance cliffs.