What is the difference of using transformer in service & direct invoke of service?

Invoking a service using a transformer, you can invoke multiple services in a single map step and every service is invoked using a separate thread thus executing parallel and improving performance.

While invoking a service as a transformer we can map only the required fields.

IF we invoke a service directly, the pipeline values of the called service will be included along with the current pipeline value and have to drop the unwanted variables explicitly.