Some flavors and ingredients will never reach there full taste or blend together properly if not cooked.
However some sauces are just mixed and not simmered. Although this does work it generally creates a more watery sauce.
The reduction of the water in the sauce to a thicker consistancy is the other main reason to simmer.
Take the same sauce and try simmering some and refrigerate the rest you will notice a difference.
It depends on your personal preference.
I you choose not to simmer, I would recommend to let the sauce sit in the fridge anyway for a couple hours to blend if not simmered.