Audio saturation is the essence of what makes analog hardware sound musical and pleasing. Driving sounds through tape, tubes, transistors, and circuits have long been an essential ingredient in great-sounding mixes. Saturation is a subtle form of distortion that adds pleasant-sounding harmonics.

Should you use saturation on every track?

That said, don't marry the idea of saturation on every track. If it's not making your mix sound any better, no matter how much you tweak the plug-in, just trash the idea and move on! Saturation can give you some interesting textures but it can also make your instruments duller.

Where does saturation go in chain?

I personally like to saturate at the end of the chain most of the time, because the I'm taking a finished sound source, that has already been EQ'd and compressed, and essentially giving it some "juice". Adding those even or odd order harmonics tends to make the sound much thicker and more exciting.