[SQLite is] a database that in full-stack culture has been relegated to "unit test database mock" for about 15 years that is (1) surprisingly capable as a SQL engine, (2) the simplest SQL database to get your head around and manage, and (3) can embed directly in literally every application stack, which is especially interesting in latency-sensitive and globally-distributed applications.
Reason (3) is clearly our ulterior motive here, so we're not disinterested: our model user deploys a full-stack app (Rails, Elixir, Express, whatever) in a bunch of regions around the world, hoping for sub-100ms responses for users in most places around the world. Even within a single data center, repeated queries to SQL servers can blow that budget. Running an in-process SQL server neatly addresses it.