It is more convenient to use serde to serialize/deserialize to some standard format like JSON or YAML than it is to write your own half-baked format and serialization/deserialization code --- even for the simplest tasks --- so you just stop doing the latter. This is a fundamental shift, and very good for your software.
"Convenience" here actually covers a lot of ground. The APIs are convenient. Boilerplate code is minimal (#[derive(Serialize)]). serde's attributes give you lots of control over the serialization/deserialization, while still being convenient to use. cargo makes importing serde into your project effortless. All of these are necessary to achieve that shift away from custom formats.
Before Rust I used to write a lot of one-off half-baked formats. Pernosco uses Rust and serde and it uses no half-baked formats. Everything's JSON, or YAML if it needs to be more human-editable, or bincode if it needs to be fast and compact and not human-readable or extensible.