Spec is great for validation. For example, we have a HTTP API built using https://github.com/metosin/reitit where we have specs for each endpoint. We have a coercion middleware that checks each request (and optionally response) conforms to a spec (and coerces things like integer ids from path to the correct type). And there's a swagger handler that automatically generates a full documentation of the API just from the route tree and specs, including samples of input and output for all routes.

Similarly you can do HTML form validation, etc.

We even use honeysql with nice helper functions that ensure that data-changing SQL queries are of the expected form. Say, you want to update just one row, you have a helper function that checks the honeysql data has a where clause with a unique column and only then runs the query. Someone should maybe blog about that sometime.

What I've found, though, is that trying to use spec like type checking is usually not a great idea. I expect better from static analysis / compile-time checking, with clj-kondo being the most promising tool atm.