Fundamentally they both need to do computation to answer query, but they are very different, a couple of highlights:
1. API service invest a lot on reliable data ingestion/loading for users, you specify data spec, data will be served with lease term reliably. It takes care of all the data sync maintenance for you.
2. It works seamlessly with real-time streaming such as Kafka for RTA. PG is not suitable for this.
3. It's distributed nature + In-memory store guarantees low-latency API service.
Hope this describes a reasonable overview to think of it.
