Along the lines of what yaten said. In a traditional sense, arrangers are usually the ones behind the actual instrumentation, as you thought, Phantoma. This might mean that they arranged the string section, or it might mean that they played/programmed the synths. A lot of the time we don't exactly know the extent of their work, as the line between composer and arranger can become blurred, but distinct traits of the arranger can often be heard. Take a listen to some of the HAL arranged songs. They certainly have their own sound in regards to synths and techniques.

Arrangement is a significant part of the music creation process.