Vince Clark of Erasure, Depeche Mode and Yaz fame, was asked once why he had an acoustic guitar in a studio, when he's primarily known as a synth keyboard player.

He replied that all his songs are first fleshed out on a simple acoustic guitar before transitioning over to more complex synth arrangements.

This resonates with my past experience as well, although there really are no rules and I'd argue so long as the end result is satisfying, any road can take you there.

For me, my best crafted work to date started fully formed only using voice and a simple little guitar.