Planetscale used to run postgres on AWS with network attached storage. So every time the DB hits the disks > it goes over the network. Need to read 4kb > network, another 4kb > network. So your latency is instead of microsecond on local storage, its miliseconds. Where a local NVME can do ... 100k 4k reads, over the network storage it does maybe 1k (just a example).

The problem is, there are not a lot of solutions to scale postgres beyond a single server. So if your DB grows to 100TB ... you have a issue as AWS does not provide a 100TB local NVME solution, only network storage.

Here comes Niki or whatever they named it. Their own alternative to Vitess (see Mysql), what is a solution that allows Mysql to scale horizontally from 1 to 1000's of servers, each with their own local storage.

So Planetscale made their own solution, so they can horizontal scale dozens, hundreds of AWS VPS with their own local storage, to give you those 100, 200, 500TB of storage space, without the need for network based storage.

There are other solutions like CockroachDB, YukubyteDB, TiDB that also allow for horizontal scaling but non are 100% postgres (and especially extensions) compatible.

Side node: The guy that wrote Vitess for Mysql, is also working on multigress (https://multigres.com/), a solution that does the same. Aka Vitess for postgres.

So yea, hope this helps a bit to explain it. If your not into dealing with DB scaling, the way they wrote it is really not helpful.