Using Feign-clients over rest-templates has number of advantages. I will list down those below.

The developer need not worry about the implementation. Just to create abstract Feign interface and few annotations - declarative principle. (If you want customized configuration, then it will hold some code)

With Spring Cloud Eureka, Ribbon client-side load-balancer will be equipped with Feign client.

No need to worry about the unit test, because there is no implementation from you to test. (Arguable)

Supports Feign annotations and JAX-RS annotations.

Highly compatible and easily configurable with Spring Cloud (Specially with Eureka server registry)

Allows Feign client configuration via @Configuration class or application properties.

Allows us to add interceptors. (Add interceptors via @Configuration or application properties. Alternatively can use Spring Cloud provided interceptors as well. Example - BasicAuthRequestInterceptor)

Hystrix support for fall-back mechanism.

Logging

Error handling

https://www.javamicroservices.com/feign-vs-resttemplate