To build up realism, it's great to apply the main colors as a quick base layer, no details. And later, add to this base with additional layers and more detail, more color, some shadows and highlights.
Is it always the case? No, you can paint realistically enough without much layering. However, for best results and a chance to tweak the painting to look like the way you want it to, nothing beats layers in watercolor.
Layers are a chance to make the shape look as you want it to, by adding shadows and highlights to the main colors. With layers, you can also increase the vibrancy and depth in a painting, thanks to watercolors being transparent.
And did you know that as a realism lover, you have a gift ? Not that of realism, as anyone can learn to master the techniques. No, chances are you have a gift for patience and adding detail, which sometimes can also make us feel like we don't belong to the watercolor world because it is often so loose.
Don't let this discourage you or feel like you need to change your style, on the contrary, I'll show you we have the power to use loose techniques to complement our realistic paintings, thanks to layering!