I think the point of ACP being an open protocol is so that other editors (e.g. VSCode, Neovim) can implement it as a receiver and integration with ClaudeCode/GeminiCLI/... would just work.
No write up, but the main reason was reusing the existing database we were comfortable deploying at the time. We were already using FDB for an online aggregation / mutation store for ad-hoc time-series analytics...albeit, a custom layer that we wrote (not RecordLayer).
When RecordLayer launched, I tested it out by building a catalog system that we could evolve and add new services with a single repository of protobuf schemas.
I leveraged FoundationDB and RecordLayer to build a transactional catalog system for all our data services at a previous company, and it was honestly just an amazing piece of software. Adding gRPC into the mix for the serving layer felt so natural since schemas / records are defined using Protobuf with RecordLayer.
The only real downside is that the onramp for running FoundationDB at scale is quite a bit higher than a traditional distributed database.
One reason would be if you're already fluent in ClickHouse's SQL dialect. Although they maintain great standard SQL compatibility, they also have a great deal of special functions/aggregates/etc that are ClickHouse specific.
Other reasons include their wide range of input formats, special table functions (e.g. query a URL).