Gouache is my favourite medium for quick colour studies. The main advantages are speed and opacity. It takes just minutes to get going, and unlike watercolours, the paints are opaque and can be painted onto any old paper, white or tinted or ordinary brown, going straight to lights and darks and middle tones without fuss or bother.
Good quality gouache paints go on smoothly and spread easily and are a pleasure to use. Only a small amount of each colour is needed on the palette. These shown here have good coverage and tinting strength -- a little goes a long way.
The paints are not waterproof which is both a plus and a minus. Any spills or splashes are easily cleaned up, unlike oil and acrylic, which need more advance preparation. Even if the paint dries accidentally in a brush it can be washed out. Also, only one brush is needed. The sable brush shown here is the only brush I've used for gouache for several years now.
The disadvantage of not being waterproof is that dried paint already on the paper may dissolve when overpainting. This doesn't bother me, I just accept the new mix or paint more thickly, depending on what I'm aiming for.
The gouache colour studies shown below, Road 1 and Road 2, both about 8.5 x 11", were each done in about 30 minutes. They are only studies and I confess I don't think of my own gouaches as finished pieces, they're just steps on the way to something else. Many painters take gouache to a high level of finish.
So, in a nutshell, for me gouache is a great way to try out ideas and to generate new ideas, because one thing does have a way of leading to another.