I was going to ask the exact same question. The title makes it sound like something I might be interested in, then I visited the page and I have no idea what it does.

After some brief reading of docs, I'm trying to write one sentence explanations. Maybe this will be helpful to you

What

Preserves is a specification and set of libraries in popular languages that lets you reliably exchange data between XML, JSON and EDN.

Who

Preserves is built for (data engineers|data framework writers) to reliably interchange data.

Why

Formats like JSON in particular are imprecise. Preserves forces you to deal with these vagaries up front

What else?

With P-Expressions you can search a preserve compliant datasource much like you would query JSON with JQ

Who Not? Who shouldn't use this

This will not help a data analyst exchange data between CSV and Excel

Useful if you have a JSON-keyed table, for example: JSON lacks a useful (standardised) equivalence relation, meaning you get weak and/or implementation-specific guarantees about how key lookup works. Equivalences were the motivation for developing Preserves: I was (and am) working on a generalized approach to messaging middleware, you might say, meaning that things like "patterns over values" and "filters" and "value-keyed tables" are all things I need to talk about. (This all comes out of RabbitMQ/AMQP thinking back in the day and my PhD-and-after work subsequently.)